Caramelized Onion Green Bean Casserole

Green bean casserole in a white baking dish topped with a golden-brown breadcrumb crust.

5 from 1 vote

Caramelized Onion Green Bean Casserole

🧅🧀 Takes the classic holiday side to the next level with caramelized onions for amazing flavor! Along with a rich cream sauce, and perfectly tender green beans, it's finished with a Parmesan and panko breadcrumb topping for irresistible crispiness. It's 100% homemade from scratch and it's the French onion green bean casserole that you'll want on your Thanksgiving and Christmas table every year!

Prep Time20

Cook Time1 30

Course: Casserole, Sides, Sides, Salads & Vegetables

Cuisine: American

Diet: Vegetarian

Keyword: caramelized onion green bean cassesrole, French onion green bean casserole, green bean casserole with caramelized onions, green bean casserole with French onions

Servings:

Calories: 250cal

Equipment

Ingredients

Onions

1 tablespoon salted butter (unsalted will work, just add a pinch more kosher salt)
3 large yellow onions halved and sliced thin vertically (sweet Vidalia onions may be used but do not sub with other types of onions)
2 teaspoons kosher salt
½ teaspoon freshly ground black pepper
1 tablespoon red wine vinegar; optional (helps cut through the richness and balances the flavor)

Filling/Sauce

Water for blanching the green beans
2 pounds fresh green beans ends trimmed and cut into 2-inch pieces (do not use frozen or canned green beans)
2 tablespoons salted butter (unsalted will work, just add a pinch more kosher salt)
8 ounces baby bella mushrooms coarsely chopped (white button mushrooms may be substituted for more mild flavor)
2 cloves garlic finely minced(unsalted will work, just add a pinch more kosher salt)
3 ½ tablespoons all-purpose flour
2 cups milk (whole or 2% recommended; don’t use skim; I've not tried plant-based)
½ cup vegetable broth (I use reduced sodium)
2 teaspoons kosher salt or to taste
¼ teaspoon freshly ground black pepper or to taste
⅛ teaspoon nutmeg; optional (be careful with this because it's noticeable even at 1/8 tsp and if you don't like nutmeg, simply omit)
1 cup Gruyere cheese freshly shredded; divided (I do not substitute; the closest sub is mozzarella but it's not the same)
¼ cup Parmesan cheese finely grated; divided

Topping

1 cup broken pieces of croutons (break them coarsely and not too fine because otherwise you'll end up with too much)
⅓ cup panko bread crumbs (do not sub with regular breadcrumbs if you want it to be lightly crispy)
½ cup Gruyere cheese freshly shredded
2 tablespoons melted salted butter (unsalted will work, just add a pinch of kosher salt)

Instructions

Onions: To a large, high-sided skillet (stainless steel or heavy bottomed preferred; I don't use nonstick skillets for this task), add the butter and heat over medium heat to melt; swirl the pan as needed.
Add the onions and cook for 2-3 minutes over medium heat. Reduce the heat to low.
Evenly season with salt, pepper, and cook the onions low and slow until deeply caramelized. It will take about an hour. Stir and toss every 5 minutes or so. Onions are ready when they are a deep and rich brown color, sweet, soft, and almost jam-like.
If you're using it, add a splash of red wine vinegar at the end, and stir to combine. It helps give dimension, depth of flavor, and helps cut through the intensity of the caramelized onions.
Caramelizing Tips - Do not rush the process because you'll just burn the onions rather than caramelize them. The highest level of heat I would recommend is medium-low heat, but if they're cooking too quickly, turn the heat down to low.
Continue working on the rest of recipe while the onions are caramelizing.
Green Beans: Trim the ends and cut them into 2-inch segments.
Bring a large stockpot of salted water to a rolling boil.
Also get a large bowl ready and fill it with ice water to create an ice bath.
Once the water is boiling, add the green beans and boil them for 3 minutes. This is called blanching. It helps tenderize the green beans without overcooking them, and preserves their bright green color
Remove them with a slotted spoon or drain and transfer them to the ice bath. This stops the cooking process so the green beans don't overcook and become mushy. Set ice bath aside while you make the filling/sauce.
Preheat: About 20 minutes before the onions will be fully caramelized, preheat your oven to 375F.
Filling/Sauce: To a separate large skillet, add the butter and heat to melt over medium heat.
Add the mushrooms and cook until soft, about 8 minutes; stir frequently.
Add garlic and evenly sprinkle the flour over top and stir to coat the mushrooms evenly. We are making a roux (butter + flour which will help thicken the sauce later).
Cook for 1-2 minutes so the flour is not longer raw. The mixture will look dry.
Slowly pour in the milk, and then the broth, while whisking continuously.
Add the salt, pepper, optional nutmeg, and whisk to combine.
Whisk nearly continuously until the sauce thickens, about 7-9 minutes.
Add the caramelized onions and stir to combine them into the sauce.
Drain and Dry: Drain the cold water from the green beans and pat dry with a clean towel or paper towels.
Layer in Dish: Spray a 9x13-inch baking dish or casserole dish with cooking spray.
Add half the green beans in the dish and evenly drizzle half the sauce over the top.
Add half the Gruyere and half the Parm.
Repeat with the remaining green beans.
Repeat with the remaining sauce, and remaining cheeses.
Set aside momentarily while you make the topping if you're making this straight through. If not, read below
Make Ahead: If you are making this in advance, stop now, cover and refrigerate for up to 24-48 hours.
When ready to bake it off, take it out of your fridge at least 1 hour before you bake it to allow it to fully come room temp. Do not bake with a fridge-cold baking dish because you will not get the results you desire!
Topping: To a small bowl add the broken croutons, panko, Gruyere cheese, drizzle the butter over, and toss lightly with a fork to coat evenly.
Crouton Tips - I just grab a handful, crush in my fist, and lightly lay the broken pieces into a measuring cup until I have 1 cup; you can also bang them on a cutting board with the blunt end of a knife or ice cream scooper, something heavy to break them lightly, not pulverized into dust.
Evenly sprinkle the topping over the green beans, sauce, and cheese that are in the casserole dish.
Bake: Bake for about 25-30 minutes, or until the topping is lightly golden brown and the casserole is bubbling on the sides, or it's as done as desired. Keep a very close eye on it and judge doneness for yourself based on how fast it's browning and bubbling.
Cool: Remove it from the oven and let it rest for about 10-15 minutes before serving. The sauce will thicken as it cools.
Serve: Serve with your favorite turkey, mashed potatoes, gravy, sweet potato casserole, stuffing, cranberries, and more.

Notes

Storage: Leftovers will keep airtight in the fridge for up to 4-5 days. I don't freeze this. The longer it sits in the fridge, the soggier the topping becomes. I reheat it gently in the microwave but you could experiment with other methods if desired.

Nutrition