Caramelized Onion and Portobello Mushroom Sliders with Fry Sauce

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Vegetarian Sliders with Fry Sauce — Meatless comfort food at its finest! The filling features a caramelized Vidalia onion, baby portobello mushrooms, and pickles.

Vegetarian Sliders with Fry Sauce — Meatless comfort food at its finest! The filling features a caramelized Vidalia onion, baby portobello mushrooms, and pickles.

Best Veggie Sliders Recipe 

These vegetarian sliders are filling, satisfying, meatless, and definitely healthier than a greasy burger or typical cookout fare. Try them for a meatless weeknight dinner if you’re trying to lighten things up.

For the filling, I caramelized a Vidalia onion in olive oil along with baby portobello mushrooms and pickles. The resulting combo is a meaty (minus the meat) sandwich that’s hearty, chewy, with subtle crunch from the onions and pickles.

I used Bread and Butter Chips from Farmer’s Garden® by Vlasic®. I love pickles, but many brands are loaded with a ridiculous amount of sodium, which is why I’ve started pickling my own vegetables

Thankfully the Vlasic pickles aren’t salt bombs and I love their sweet-and-tangy zip. They’re artisanal-quality, premium pickles, made with honest ingredients and no artificial flavors. They’re made with deliciously ripe cucumbers, whole garlic cloves, slices of red bell pepper, rounds of carrots, fresh herbs and spices, and hand-packed in glass jars.

Vegetarian Sliders with Fry Sauce — Meatless comfort food at its finest! The filling features a caramelized Vidalia onion, baby portobello mushrooms, and pickles.

Before filling the sliders, I coated the bread with fry sauce. If you’ve never had it, you’ve got to try it. Sometimes fry sauce is called Utah sauce or Utah fry sauce, but I waitressed in a dive bar in Minnesota in college and this is the sauce we served with wings and waffle fries so it’s not just a Utah thing.

It’s two parts ketchup to 1 part mayo (use vegan mayo) with a splash of pickle juice. Now you have a use for pickle juice rather than just tossing it.

The sauce is excellent on fries, but it makes a great sandwich spread and even for mayo-haters, it’s not overly mayo-ey.  I like a little food with my bowl of sauce or dip and can devour a batch on about 7 fries.

I loved this filling mixture and didn’t even need buns. My family didn’t even realize it was meatless. Comfort food at its finest.

Vegetarian Sliders with Fry Sauce — Meatless comfort food at its finest! The filling features a caramelized Vidalia onion, baby portobello mushrooms, and pickles.

Ingredients for Vegetarian Sliders

To make these onion and mushroom sliders, you’ll need: 

  • Sweet Vidalia onion
  • Olive oil
  • Baby portobello mushrooms 
  • Bread and butter pickles
  • Pickle juice
  • Slider buns or Hawaiian rolls
  • Ketchup 
  • Mayonnaise or Miracle Whip
  • Onion powder

Vegetarian Sliders with Fry Sauce — Meatless comfort food at its finest! The filling features a caramelized Vidalia onion, baby portobello mushrooms, and pickles.

How to Make Vegetarian Sliders

These easy vegetarian sliders come together quickly and require minimal assembly. Here’s how the filling is prepared and the sliders are assembled: 

  1. Make the filling: Caramelize the onionsin a non-stick skillet over medium-high heat.  It takes about 10 minutes for my onions to caramelize and I cook them covered to save time. 
  2. Once the onion has caramelized, add the mushrooms, pickles, pickle juice, a little more oil, and allow mixture to cook for about 10 minutes, stirring every couple of minutes.
  3. Make the fry sauce: In a medium bowl, combine all ingredients and whisk until smooth.
  4. Assemble the sliders: Generously spread fry sauce on both sides of slider buns.
  5. Fill each bun generously with the onion mixture and serve. 

Vegetarian Sliders with Fry Sauce — Meatless comfort food at its finest! The filling features a caramelized Vidalia onion, baby portobello mushrooms, and pickles.

Can This Recipe Be Made in Advance? 

If you plan on making these vegetarian sliders for a party, you can make the filling up to 3 days in advance and the fry sauce up to 1 week in advance. When you’re ready to serve, simply reheat the filling in a skillet and then assemble the sliders as instructed. 

What to Serve with Veggie Sliders

To make the portobello sliders a meal, serve them with one or more of the following sides: 

Vegetarian Sliders with Fry Sauce — Meatless comfort food at its finest! The filling features a caramelized Vidalia onion, baby portobello mushrooms, and pickles.

Tips for the Best Vegetarian Sliders

Have patience when cooking the mushrooms and onions. The filling takes about 20 minutes total to cook — do not crank up the heat to save time as that will result in a less rich filling. 

However, you can always pop the lid on your skillet to safely speed up the cook time. That’s what I did! 

Leftover fry sauce is delicious slathered on wraps and sandwiches, and can also be used as a dipping sauce. 

 

Pin This Recipe

Yield: 6

Vegetarian Sliders with Fry Sauce

Vegetarian Sliders with Fry Sauce

Meatless comfort food at its finest! The filling features a caramelized Vidalia onion, baby portobello mushrooms, and pickles.

Prep Time 10 minutes
Cook Time 20 minutes
Total Time 30 minutes

Ingredients

Sliders

  • 1 medium sweet Vidalia onion or yellow onion, peeled and sliced
  • 2 to 3 tablespoons olive oil, divided
  • 4 ounces sliced baby portobello mushrooms
  • 1 cup diced Vlasic Farmer’s Garden Bread and Butter Chip pickles
  • 1 teaspoon pickle juice
  • 6 slider buns (or Hawaiian rolls)

Fry Sauce

  • 1/2 cup ketchup
  • 1/4 cup mayo (or Miracle Whip, use vegan mayo to keep vegan)
  • about 3 teaspoons pickle juice, or to taste
  • pinch onion powder, optional and to taste

Instructions

Filling:

  1. Heat a non-stick skillet over medium-high heat, add the onion, drizzle with about 1 tablespoon oil, and allow onion to caramelize, stirring every couple of minutes, or as needed. It takes about 10 minutes for my onions to caramelize and I cook them covered to save time. Lower the heat if onion is browning too fast.
  2. After onion has caramelized, add the mushrooms, pickles, pickle juice, 1 to 2 tablespoons oil or as necessary, and allow mixture  to cook for about 10 minutes, stirring every couple of minutes, or as needed. I cook covered to save time. After about 10 minutes, or after vegetables have softened and wilted, turn off the heat; set aside.
  3. While vegetables are cooking, make the fry sauce.

Fry Sauce:

  1. In a medium bowl, combine all ingredients and whisk until smooth.
  2. Generously spread fry sauce on both sides of slider buns.
  3. Fill each bun generously with the onion mixture and serve.

Notes

Storage: Sliders are best warm and fresh although extra filling will keep airtight in the fridge for up to 3 days. Extra fry sauce will keep airtight in the fridge for up to 1 month.

Nutrition Information:

Yield:

6

Serving Size:

1

Amount Per Serving: Calories: 541Total Fat: 46gSaturated Fat: 22gTrans Fat: 1gUnsaturated Fat: 21gCholesterol: 85mgSodium: 797mgCarbohydrates: 30gFiber: 3gSugar: 11gProtein: 6g

More Party Sliders Recipes: 

Ham and Cheese Sliders — Baked juicy ham and Swiss nestled in soft Hawaiian rolls and brushed with a buttery Dijon, onion, and poppy seed topping! Fast, easy, and a party favorite!

Pepperoni Pizza Sliders — Pizza by way of supremely cheesy sliders!! EASY, ready in 15 minutes, and perfect for parties because everyone LOVES them! Totally IRRESISTIBLE!!

Ranch Chicken and Swiss Sliders — Tender chicken coated in ranch, nestled in soft rolls with Swiss and a buttery Dijon topping! Fast, easy, and everyone loves them! Perfect for parties, game days, and snacks!

Beer and Bourbon Barbecue Chicken Sliders – Chicken is cooked with beer and bourbon before being soaked in bbq sauce! So juicy, flavorful, easy, and ready in 25 minutes!

Hawaiian Ham and Swiss Sliders — Grilled pineapple slices brushed with a soy and ginger marinade add a Hawaiian-inspired twist to these irresistible sandwiches! Juicy ham, melted Swiss cheese, and melted butter are impossible to resist!

Hawaiian Ham and Cheese Sliders - Grilled pineapple slices brushed with a soy and ginger marinade add a Hawaiian-inspired twist to these irresistible sandwiches! Juicy ham, melted Swiss cheese, and melted butter are impossible to resist! FAST, EASY, perfect as a game day or party appetizer, snack, or quick weeknight dinner!



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Welcome to AverieCooks! Here you’ll find fast and easy recipes that taste amazing and are geared for real life. Nothing fussy or complicated, just awesome tasting dishes everyone loves!

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Comments

  1. We made them for this morning’s breakfast and they turned out really great! The sauce amazingly compliments all the flavours. Thank you :)

  2. Made this for dinner and it was devoured by all. Even my mushroom hating kid loved it (I don’t lie, I told her what was in it). I was very reluctant to add pickles but followed your directions. Sauce also great. Not so quick, carmelizing onions took a while, but fantastic results. Thank you, Averie!

    1. The fact that you got a mushroom hating kid to love this, that’s wonderful. And yes the pickles add a nice touch and so glad you like the sauce, too!

  3. Oh Averie, these look incredible! The sweet onions, the crunchy pickles, the gorgeous filling… So why oh why do I have to be allergic to mushrooms??? Not fair! Is there anything I could substitute instead? Because these look way too good to pass by! :) We haven’t had a 4th of July party for as long as I can remember, but the past few years my guy and I have camped out at the local park to watch the fireworks. Despite being a tiny town, they actually choreograph them to music — it’s so fun!

    1. You could just omit them or use lentils, sweet potatoes, another vegetable – whatever you’d like!

      1. Caramelized onions and sweet potatoes — now that sounds like perfection! Hard to beat naturally healthy sweetness, and my waistline doesn’t mind that as much as an extra cookie (or four)! ;)

  4. fry sauce??!!??!! I’ve got to make these little cuties just to try that sauce! Looks amazing! Love a little savory in the mix!

  5. Delish! Fry sauce sounds intriguing; I wonder if I could swap mustard for ketchup? (I put Dijon mustard on just about everything!) :D

    1. It would definitely give it a different flavor but I really don’t think there’s a wrong way to mix condiments together :)

  6. I have never had fry sauce this is a must try…these sliders look so good I love small food for some reason it just tastes better.

    1. It totally DOES taste better and the fry sauce is crazy good and I want it on everything. Something so simple but so darn good!

  7. Yum! Love mushrooms and onions! I’ve heard of fry sauce just never made it. Now might be the time! As for the pickle juice I use it to marinade chicken for sandwiches…supposedly that’s what they do at chick-fil a (I’ve never even seen one of those tho’).

    1. I think any acid like vinegar, lemon juice, pickle juice is great for marinating things – really helps kickstart that process!

  8. Mmmmmm!!!!! I LOVEEEE the sweet crunch of caramelized onions and the meaty texture of these mushrooms is making me drool! And as if that wasn’t enough, there’s this delish mayo and ketchup in fry sauce! SALIVATING!!!!!

  9. Love this meatless burger and that fry sauce. Oh my, I think I would find any excuse to slather that on everything – sounds amazing!

  10. These do look like they will make my stomach so much happier than a greasy burger! Pretty excited about vidalia onion season down here in GA!

  11. I could eat that fry sauce on evvvvverything. Correction… I do eat that sauce on everything! Looks delicious.

    1. Love the name of your blog! And so glad you know Fry Sauce, and sounds like you know it well :) Me too!

  12. Mushrooms and onions are soooo good together. I never would have thought to add bread and butter pickles (love those too) so I am intrigued and just have to try this! I’ve never heard of fry sauce either but have used ketchup and mayo together–and now I know what it’s called when you put a little pickle juice in. I am loving these savory recipes of yours! I also like saucy food so this sounds like something I could use on many things.

    1. The tanginess of the pickles with the onions and shrooms, Paula, so good! Make the mixture and even just mix in some brown rice, lentils, beans, chickpeas, whatever! It could be like a casserole/side-ish or even main dish. I think you’d love it!

      And the fry sauce. So simple but so, so, so good. You have to make it. Then you will want to slather it on everything. Saucy food, yes. Never dry anything, ever!

      1. I tried this for lunch today and went with your suggestion to add lentils. Perfect meal!! I really love the bread and butter chips in the mix–what made you think of it? It all just works and the fry sauce ties it together. I could eat this again for dinner.. but I’ll try to save the leftovers so Jon can try it when he gets home tonight. I think the fry sauce would be great with parsnip fries.

      2. What made me think of it is when Vlasic contacted me and wanted a recipe using their new pickles. So I had to think pickles, what goes with pickles, like how could I elevate pickles rather than just plopping them on a sandwich. So I decided to stir fry them. And with what…hmmm, onions, shrooms, and then from there (kind of after the fact) realized that lentils, chickpeas, brown rice, wild rice, so many things would be really good in that mixture! Glad the lentils worked and I hope Jon gets a taste but I would not blame you AT ALL for that not exactly happening :) I had intended to share much of this mixture with the family but…that didn’t happen. I went at it with a fork straight out of my leftovers container. Whoops :)

  13. First of all, anything with mayo is overly mayo-y. :P

    Second, I LOVE grilled portobellos and if we lived closer, you would for sure be invited to a BBQ here. BIG TIME. I haven’t grilled mushrooms in forever because my friend who always comes to my BBQs doesn’t like them. That needs to change though. Next time I’m at the store, I’m getting them, and eating them ALL myself! With lots of BBQ sauce or ketchup/mustard/nooch sauce.

    Lastly, caramelized onions are such a delicacy that are so underrated. I used to make a huge batch on weekends and just eat them with a fork, cold, straight from the fridge. Delicious. You can make them in the slow cooker, too, to make them even easier!

    1. I wrote that mayo comment JUST for YOU! And seriously though, you will love this sauce. I know you think you hate mayo but honestly this is a doctored up ketchup lover’s dream (and that describes you!)

      I wish I had a backyard, a grill. I wish I had…well, a midwest summer night’s bbq is the best thing ever. Totally wish I could join you!

      caramelized onions are such a delicacy that are so underrated = Yes so true! The mixture itself with the shrooms, too, so good. You could mix in some brown rice, lentils, beans, chickpeas, whatever and it could be like a casserole/side-ish or even main dish. So many ways to go – or you could just eat the caramelized onions by themselves which I’d be fine with :)

  14. Caramelized onion makes everything taste just a little bit better…Those look flippin amazing! Great job Averie :)

  15. I’ve been drooling over these since I saw them on IG earlier!! I think the simplicity of this is genius, and I bet these are completely amazing!!

    1. I have way more of a sweet tooth than a savory tooth but was totally enjoying the flavors in this so much!

  16. Those are my favorite pickles – they’re sweet. And caramelized Vidalia onions. And mushrooms. This needs to be my next dinner!

  17. Girl!!! We are so on the same page! I posted portobello burgers last week and sliders today! Great minds think alike!! These look absolutely amazing Averie and love the addition of the fry sauce.. killer!

  18. Anything with caramelized onions is a winner in my book. This looks soooo good. Pinning to save for later! :-)

  19. This has comfort food written all over it, Averie! I’m loving this savory side of yours :). And how about that beautiful fry sauce?? Can’t believe I’ve never heard of it before. When I was a kid, I loved dipping my fries first in ketchup and then in mayo, but never mixed the two. So I know I would be all over this!!