This homemade French Onion Green Bean Casserole swaps the traditional fried onions for rich, golden brown caramelized onions. With melty gruyere cheese and a buttery crouton topping, it’s a delicious collision between classic green bean casserole and French onion soup!
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The sides are the perfect place to have a little fun at Thanksgiving.
While a traditional Thanksgiving Turkey might be the centerpiece of the meal, let’s be honest: it’s all about the sides!
My family’s menu always consists of Chestnut Stuffing, Glazed Sweet Potatoes, and Mom’s Crockpot Mashed Potatoes. This year we’re adding this unique twist on green bean casserole to the menu, and I am thrilled.
I took my go-to homemade Green Bean Casserole and gave it the French Onion Soup treatment: caramelized onions, a crispy crouton topping, and gruyere cheese. It is creamy, cheesy, and absolutely HEAVENLY.
If you bring a side dish to Thanksgiving or Friendsgiving this year, make it this one. Your friends and family will be talking about it long after the meal!
Are you hosting Thanksgiving this year? Be sure to download my Less-Stress Thanksgiving Guide, complete with a menu and realistic Thanksgiving prep guide!
Key Ingredients
You’ll find the full list of ingredients in the recipe card below, but here are some notes to keep in mind.
- Green Beans. FRESH green beans are essential for a delicious green bean casserole. Frozen is the next best option. DO NOT use canned green beans (they are one of the reasons so many people think they don’t like green bean casserole!).
- Milk + Butter + Flour + Mushrooms. These create the creamy sauce. No canned cream of mushroom soup for us! It’s easy to make yourself and the taste is sooooo worth it.
- Gruyere Cheese. Emmental is a great substitute.
- Caramelized Onions. Sweet, golden, and tender, they’re delectable layered with the green beans and cheese.
- Croutons. For the crispy, French onion soup-inspired topping.
How to Make French Onion Green Bean Casserole
Caramelize the Onions. Cook them in butter over medium heat for 5 minutes, then reduce the heat to low, season, and cook until the onions are dark brown and jammy. This can take up to 120 minutes. (Between you and me, people who tell you to caramelize onions in 10 minutes don’t know what REAL caramelized onions are.)
Blanch the Beans. Drop them in a pot of boiling water for 3 minutes, then plunge them into an ice bath. Drain and pat dry—wet beans will make your casserole watery!
Cook the Mushrooms. Melt butter in a pan, then brown the mushrooms. They’ll be done when they release their juices and then those juices cook off. Sprinkle flour over the mushrooms and cook for a minute or two.
Make it Creamy. Slowly add the milk, then increase the heat to medium-high. Cook until the sauce thickens, then season with salt and pepper and add the nutmeg.
Assemble. Add half of the green beans to a baking dish, followed by half the mushrooms, half the onions, and half the cheese. Repeat!
Top It. Mix the croutons and Parm with melted butter and sprinkle this delightful mix over the top, along with the remaining cheese.
Bake. Pop your French onion green bean casserole in a 375ºF oven for 25 to 30 minutes, or until it’s melty and nicely browned on top. Rest for 10 minutes to let the sauce thicken up a little, then ENJOY!
Essential Thanksgiving Recipes
You probably have your own faves all lined up for the BIG DAY, but here are some of mine:
- The secret to juicy turkey: a Dry Brine or Wet Brine
- Turkey Gravy
- Sweet Potato Casserole with Pecan Topping
- Cornbread Stuffing
- Garlic Mashed Potatoes
- Bourbon Pecan Pie
- Buttermilk Pie
How to Prep this Recipe Ahead
- Up to 3 Days in Advance. Caramelize the onions and grate the cheese.
- Up to 1 day in Advance. Blanch and refrigerate the green beans.
- To Assemble Ahead. The day before, layer everything into your casserole dish except for the topping; cover and refrigerate. When ready to bake, remove the dish from the refrigerator and let come to room temperature. Uncover, add the topping, then bake as directed.
French Onion Green Bean Casserole
Video
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Ingredients
- 4 medium yellow onions halved lengthwise and thinly sliced
- 5 tablespoons unsalted butter divided
- 2 teaspoons kosher salt divided
- 1 teaspoon ground black pepper divided
- 2 ½ pounds green beans trimmed and cut into 1 ½-inch pieces
- 8 ounces baby bella mushrooms sliced
- 2 tablespoons all-purpose flour
- 2 ½ cups 2% milk plus additional as needed
- ⅛ teaspoon ground nutmeg
- 1 cup croutons crushed
- ¼ cup freshly grated Parmesan cheese
- 1 ½ cups shredded Gruyere cheese divided
Instructions
- Caramelize the onions: In a large, wide saucepan, melt 1 tablespoon of the butter over medium heat. Add the onions and cook 5 minutes. Reduce the heat all the way to low and stir in 1 teaspoon salt and 1/2 teaspoon pepper. Continue to cook over super low heat, stirring periodically, until the onions cook down significantly and are golden and caramelized – depending upon your pan, about 90 to 120 minutes. If at any point the onions start to stick, stir in water 1 tablespoon at a time to loosen. Remove to a plate.
- Place a rack in the center of your oven and preheat to 375°F. Coat an 8×10-in or similar casserole dish with nonstick spray.
- Bring a very large pot of water to a boil. Add the green beans and cook just until crisp-tender, about 3 minutes. Plunge into an ice bath to stop the cooking, then drain and transfer to the towel. Pat dry.
- Heat 2 tablespoons butter in a large, wide saucepan over medium heat (if you want to use the one you caramelized the onions in, wipe it out with a paper towel first). Add the mushrooms. Cook until the mushrooms brown, about 10 minutes.
- Sprinkle the flour over the top. Cook, stirring constantly, for 1 to 2 minutes, until all of the flour turns golden and no white bits remain. The pan will seem very dry.
- Slowly add the milk a few splashes at a time, stirring between each addition to prevent lumps from forming. Increase the heat to medium high. Cook and stir, allowing the sauce to bubble, running a wooden spoon or spatula along the bottom of the pan until the sauce reduces and thickens to resemble a creamy gravy, about 8 minutes.
- Stir in the remaining 1 teaspoon salt, 1/2 teaspoon pepper, and nutmeg. If at any point the sauce gets too thick, stir in a little more milk as needed.
- Spread half of the green beans into the prepared dish. Spoon half of the mushroom sauce over the top, then sprinkle with half the caramelized onions, and half of the cheese. Add the remaining green beans, remaining sauce, and remaining caramelized onions.
- Melt the remaining 2 tablespoons butter in a small bowl, then stir in the croutons and Parmesan. Sprinkle on top. Sprinkle with the remaining cheese.
- Bake until the casserole is hot and bubbly and the cheese is melted and the top is lightly browned, about 25 to 30 minutes. Let stand 10 minutes, then serve.
Notes
- MAKE AHEAD. Caramelize the onions and grate the cheese up to 3 days in advance. Blanch the green beans up to 1 day in advance.
- TO STORE. Refrigerate for up to 4 days. Reheat in a 350°F oven.
Nutrition
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We absolutely loved it!
Thank you Sarah!
Really simple to make and a huge crowd pleaser. It was delicious! Will definitely make it again!
Great to hear! Thanks Daniel!
Thoughts on using emmentaler instead of gruyere? Sometimes it’s hard to find!
Hi Laura, it should work well! Enjoy!
What brand & were they plain or seasoned breadcrumbs? This looks awesome !
Hi Debra, no particular brand, whatever you can find in your store. I used plain. Enjoy!
Would seasoned canned green beans work?
Hi Jennifer, I wouldn’t recommend to use them.
Help! What brand/ kind of croutons should I buy?
Hi Keisha, any brand of plain croutons would work! Enjoy!
Hi Erin. I love green beans, but dislike mushrooms. Is there anything I can use in place of the mushrooms in this recipe? Thanks!
Hi Agnes, you can just leave them out. Enjoy!
Hi! I only have whole milk. I’m assuming this is fine to substitute for the 2 percent? I hope! Thank you.
Yes! Enjoy Stephanie!
So excited to make this for Thanksgiving Day! We are traveling about an hour to our destination and wanted to get your advice on preassembling this dish. Can I assemble the whole dish ahead or should I prepare all of the ingredients and then assemble there? Thank you so much! Donna
Hi Donna, we have some tips in the post that tells you the best way to prep the casserole ahead of time in steps and then to assemble later. Hope this helps!
Can I still use crispy fried onions on the top ?
Hi Lisa, it should work but this version is top-notch and worth the extra step!
they were out of fresh. So I got frozen. Any tips?
Hi Katy, I believe you can simply thaw and then drain them. Let us know how it goes if you decide to experiment!
Hi Erin, I did you turkey brine and will be doing your turkey recipe this Thanksgiving as well ! Can’t wait 😊
Question : do you think I can make this the day before thanksgiving and reheat ?
Hi Jennifer, yes that should work!
Hi I’m a little confused on the crouton part. So stir in croutons from the bag into melted butter then layer on top? Thanks in advanced!!!
Hi Erika, you’ll crust the slightly to make 1 cup and add to the butter. Then you will layer it on top. Hope this helps! Enjoy!
If I prep the onions in advance, do I need to reheat them the day I make the casserole or just layer in cold?
Hi Landra, I’d probably let them come to room temperature but making them the day before should work!
Do you cover the onions when cooking them?
Hi Sarah, no need to cover! Enjoy!
Hi! Can I use swiss cheese?
Hi Gabriela, that should be ok. Enjoy!
I don’t see where it says what temp to bake at?
Hi Amy, it’s in step 2. 375°F. Enjoy!
Hi! It’s step 2 in the recipe card: 375°F.
Am I reading this right that you cook the onions first for 90-120 minutes alone? That seems like a really long time
Hi Brianna! Yes, so worth it! Enjoy!
I know it’s crazy but that is how long it takes to gently caramelize a whole bunch of onions! You could cook them for less time at a higher temp; they’ll be sauteed but not caramelized. Up to you and how much time you have.
Made this as a Thanksgiving side dish last night and it was a huge hit! So delicious!!! And this is coming from someone who does not like green beans! =)
So great to hear, thank you Monica!
This was an instant hit! I’ll gladly make this every year forever! Chefs kiss, perfect!
Wonderful! Thanks for the feedback, GDsam!
The BEST green bean casserole I’ve ever had! The Gruyère cheese, creamy mushroom sauce, and crouton topping really makes this recipe. I’m planning on making this every Thanksgiving from here on out.
So happy to hear it, thanks so much for sharing!!!!
Can’t tell you all how much of a hit this was for my partner and I! We absolutely loved it!
So great to hear! Thank you Ivan!
We absolutely loved it! Prepped the day before and then baked day of Thanksgiving!
Yay! Thank you Brenna!
I love caramelized onions and mushrooms so this dish was a home run for me! The buttery croutons and gruyere cheese were perfect. To make it more appealing to everyone in my family I may use fewer onions next time.
So glad to hear, thank you JM!
This was probably the best green bean casserole I have ever made. I was only responsible for 2 dishes this Thanksgiving and I have to say these outshined every other dish.
So glad to hear, thank you Shane!
We absolutely loved it! Will be on the Thanksgiving and Christmas table again and again. I will make a double recipe!! There were no leftovers!
So happy to hear, Jocelyn! Thank you!
I have made this twice now and everyone has loved it!! I’ve made it with canned green beans and then fresh green beans another time, both good but fresh is better. We made it with gluten free flour and it turned out great, also used crispy onions on top instead of croutons. It’s a new staple for holidays now!
So glad to hear, Shannon! Thank you for the feedback!