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. 

 

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5 from 1 vote

Vegetarian Sliders with Fry Sauce

By Averie Sunshine
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
Servings: 6
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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

  • ½ cup ketchup
  • ¼ 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:

  • 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.
  • 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.
  • While vegetables are cooking, make the fry sauce.

Fry Sauce:

  • In a medium bowl, combine all ingredients and whisk until smooth.
  • Generously spread fry sauce on both sides of slider buns.
  • 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

Serving: 1, Calories: 541kcal, Carbohydrates: 30g, Protein: 6g, Fat: 46g, Saturated Fat: 22g, Polyunsaturated Fat: 21g, Trans Fat: 1g, Cholesterol: 85mg, Sodium: 797mg, Fiber: 3g, Sugar: 11g

Nutrition information is automatically calculated, so should only be used as an approximation.

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!

About the Author

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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Please note: I have only made the recipe as written, and cannot give advice or predict what will happen if you change something. If you have a question regarding changing, altering, or making substitutions to the recipe, please check out the FAQ page for more info.

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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!