Crispy Onion Green Bean Casserole with Mushroom Roux
A modern take on the classic casserole combining roasted crispy onions, sauteed mushrooms, and creamy roux-based sauce. Fresh green beans stay tender while the toasted breadcrumb-onion topping adds crunch and savory depth. Serve this as a comforting side dish for weeknight dinners or holiday gatherings. The combination of umami from mushrooms, garlic, and parmesan simple vegetables into a satisfying main accompaniment.
Ingredients
- 2 yellow onions, thinly sliced
- 1 tablespoon olive oil
- ½ cup breadcrumbs
- ½ cup all-purpose flour
- ⅓ cup all-purpose flour
- ½ teaspoon salt
- 1 teaspoon salt
- 2 pounds fresh green beans, trimmed
- 2 tablespoons unsalted butter
- 2 tablespoons unsalted butter
- 1 pound baby bella mushrooms, sliced
- ½ teaspoon black pepper
- 2 cups vegetable broth
- 2 teaspoons garlic, minced
- ½ cup grated parmesan cheese
Instructions
- 1
Preheat oven to 425F. Spray large baking sheet with nonstick spray.
- 2
Toss sliced onions with olive oil, breadcrumbs, 1/2 cup flour, and 1/2 teaspoon salt. Spread on baking sheet and bake 20 minutes until golden and crispy.
- 3
Melt 2 tablespoons butter in large skillet over medium heat. Add mushrooms, 1 teaspoon salt, and 1/2 teaspoon pepper. Saute 8 minutes until mushrooms darken and shrink. Transfer to bowl.
- 4
In same skillet, melt remaining 2 tablespoons butter. Whisk in 1/3 cup flour to form roux. Gradually whisk in broth and simmer 5 minutes until thickened.
- 5
Spray 9x13-inch baking dish with nonstick spray. Combine mushrooms, green beans, half the onion mixture, parmesan, and garlic in large bowl. Transfer to baking dish.
- 6
Pour roux over vegetables. Reduce oven to 350F. Cover with foil and bake 30 minutes.
- 7
Uncover, top with remaining crispy onion mixture. Bake uncovered 10 minutes until onions are heated and browned.
Tips
Make the crispy onion topping a day ahead and store in an airtight container. Add just before final baking to maintain maximum crunch.
Don't skip the 5-minute simmer for the roux - it allows the flour to fully cook and prevents a raw flour taste in the sauce.
Good to Know
Cover and refrigerate up to 4 days. Reheat covered at 350F for 15-20 minutes until warmed through.
Assemble casserole through step 5 the day before. Cover and refrigerate. Add 5-10 minutes to baking time if baking from cold.
Serve hot as a vegetable side dish alongside roasted chicken, turkey, or beef. Also works as a vegetarian main with crusty bread and salad.
Common Mistakes
Don't skip prepping green beans - leave them whole or cut into 2-inch pieces to maintain texture
Don't stir the mushrooms constantly - let them sit to develop deeper color and caramelization
Don't add onion topping before final 10 minutes or it will become soggy instead of crispy
Substitutions
FAQ
Can I make this with frozen green beans?
Yes, use thawed frozen green beans drained well. They cook faster so reduce the covered baking time to 20-25 minutes. Fresh beans hold their texture better, but frozen works in a pinch.
What if my roux is too thick or too thin?
Too thin: whisk in a flour-butter paste and simmer longer. Too thick: whisk in broth gradually. The sauce should coat the back of a spoon lightly.
Can I freeze this casserole?
Yes, freeze unbaked or baked. Unbaked: freeze after assembly, add 15-20 minutes to baking time. Baked: cool, freeze up to 3 months, reheat covered at 350F for 25-30 minutes.