Green Bean Classic Casserole (Printable)

Tender green beans baked with mushroom sauce and crispy onions for a comforting side dish everyone enjoys.

# What You'll Need:

→ Vegetables

01 - 1 lb fresh green beans, trimmed and cut into 2-inch pieces (or 1 lb frozen, thawed)
02 - 1 small yellow onion, finely chopped
03 - 2 cloves garlic, minced
04 - 8 oz cremini or button mushrooms, sliced

→ Sauce

05 - 2 tbsp unsalted butter
06 - 2 tbsp all-purpose flour
07 - 1 cup whole milk
08 - 1/2 cup low-sodium vegetable broth
09 - 1/2 cup sour cream
10 - 1/2 tsp salt
11 - 1/4 tsp black pepper
12 - 1/4 tsp ground nutmeg

→ Topping

13 - 1 1/2 cups crispy fried onions (store-bought or homemade)

# How-To Steps:

01 - Preheat the oven to 350°F.
02 - Bring a large pot of salted water to a boil. Add green beans and cook for 4 to 5 minutes until bright green and just tender. Drain and immediately transfer to ice water to stop cooking. Drain again and set aside.
03 - In a large skillet over medium heat, melt butter. Add chopped onion and cook for 2 to 3 minutes until softened. Add garlic and mushrooms; cook for 5 minutes until mushrooms are tender and any liquid has evaporated.
04 - Stir in flour and cook for 1 minute, stirring constantly. Gradually whisk in whole milk and vegetable broth. Cook, stirring, until the sauce thickens, about 3 to 4 minutes.
05 - Remove skillet from heat. Stir in sour cream, salt, black pepper, and nutmeg until smooth.
06 - Add the drained green beans to the sauce and stir to combine evenly.
07 - Transfer the mixture to a greased 2-quart baking dish. Sprinkle 1 cup of crispy fried onions evenly over the top.
08 - Bake for 25 minutes until bubbling.
09 - Remove from oven, top with remaining fried onions, and bake an additional 5 minutes until golden brown.
10 - Allow to rest for 5 minutes before serving.

# Expert Advice:

01 -
  • It comes together in under an hour and tastes like you fussed over it for days.
  • The combination of creamy sauce and crispy onion topping hits that perfect texture balance that makes people reach for seconds.
  • You can prep it ahead and slide it into the oven right before guests arrive, which is basically a recipe superpower.
02 -
  • Blanching the green beans first keeps them from staying too firm in the casserole—they finish cooking in the oven and stay tender, not crunchy.
  • Adding the sour cream off heat is non-negotiable; otherwise you'll end up with little broken bits instead of a smooth, creamy sauce.
  • Save some crispy onions for the very end of baking so they don't turn into mush; the texture contrast is what makes this dish sing.
03 -
  • Don't skip the ice water bath for the green beans—it stops the cooking at exactly the right moment and keeps them from turning that sad olive-green color.
  • Taste the sauce before it goes into the baking dish and adjust the salt and pepper then, because baking concentrates flavors and you can't fix it after.
  • Let the finished casserole rest for 5 minutes before serving so the sauce sets slightly and everything stays on the plate instead of sliding around.
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