Can I assemble this green bean casserole the night before Thanksgiving?

Yes, and you should. Assemble the beans and mushroom sauce in the baking dish through the baking step, cover tightly, and refrigerate up to 2 days. Bake straight from the fridge with 10 to 15 extra minutes, and add the crispy shallots only after baking so they stay crunchy.

How do I keep the crispy shallot topping from going soggy?

Fry the shallots to a deep amber, drain them well on paper towels, and store them at room temperature in an airtight container with a paper towel inside. Add them to the casserole only after it comes out of the oven. Refrigerating fried shallots is what makes them go soft.

Can I use frozen green beans instead of fresh in this casserole?

You can, but thaw them completely and pat them very dry first, since excess water thins the sauce. Skip the blanching step and reduce the final bake by about 5 minutes. Fresh beans give a brighter color and snappier bite, so use them when Thanksgiving quality matters.

Why does my mushroom cream sauce taste floury or look lumpy?

A floury taste means the roux did not cook long enough, so stir the flour in the butter for a full 2 minutes until it smells nutty before adding liquid. Lumps come from adding the broth too fast, so pour slowly while whisking constantly. If lumps still form, strain the sauce before adding the beans.

Can I make this green bean casserole dairy free?

Yes. Use olive oil instead of butter, full fat coconut milk or unsweetened oat cream instead of heavy cream, and skip the Gruyere or use a dairy free melting cheese. The browned mushrooms, Dijon, and thyme carry so much flavor that most guests will not notice the swap.

Green Bean Casserole From Scratch (2025)

Ingredients

Scale

2 pounds fresh green beans, trimmed
3 tablespoons unsalted butter, divided
12 ounces cremini mushrooms, thinly sliced
1 large yellow onion, finely diced
3 garlic cloves, minced
1/4 cup all-purpose flour
1 1/2 cups low sodium vegetable broth
1 1/2 cups heavy cream
1 teaspoon Dijon mustard
1/2 teaspoon dried thyme
1/4 teaspoon freshly grated nutmeg
1 cup shredded Gruyere cheese, divided (about 4 ounces)
1 1/2 teaspoons kosher salt, plus more to taste
1/2 teaspoon freshly cracked black pepper
3 large shallots, very thinly sliced (for the topping)
1/3 cup all-purpose flour (for the topping)
1/2 teaspoon kosher salt (for the topping)
1 cup vegetable oil, for frying (for the topping)

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Instructions

Preheat the oven to 375°F. Lightly grease a 9×13 inch baking dish with butter and set it aside.
Bring a large pot of generously salted water to a rolling boil. Add the green beans and blanch for 4 to 5 minutes, until bright green and just tender. Drain, plunge immediately into a large bowl of ice water, then drain again and pat very dry with clean towels.
Make the crispy shallots: toss the sliced shallots with 1/3 cup flour and 1/2 teaspoon salt, separating the rings. Heat the vegetable oil in a deep skillet to 350°F. Fry the shallots in small batches until deep golden, about 2 minutes per batch. Lift out with a slotted spoon, drain on paper towels, and sprinkle lightly with salt.
Melt 2 tablespoons of butter in a large, deep skillet over medium high heat. Add the mushrooms in a single layer and cook undisturbed for 3 minutes, then stir and continue cooking until deeply golden and their liquid has evaporated, 8 to 10 minutes total.
Add the diced onion to the mushrooms and cook, stirring, until soft and lightly golden, about 5 minutes. Stir in the garlic and cook 30 seconds more, just until fragrant.
Sprinkle 1/4 cup flour over the mushroom mixture and stir constantly for 2 minutes to cook out the raw flour taste. Slowly pour in the vegetable broth while whisking, then pour in the heavy cream in a steady stream, whisking smooth.
Bring the sauce to a gentle simmer and cook 4 to 5 minutes, stirring often, until thick enough to coat the back of a spoon. Stir in the Dijon mustard, thyme, nutmeg, and half of the Gruyere. Season with 1 1/2 teaspoons kosher salt and the black pepper, then taste and adjust.
Add the blanched green beans to the skillet and fold gently until every bean is coated in sauce. Transfer the mixture to the prepared baking dish and spread it evenly.
Sprinkle the remaining Gruyere over the top. Bake for 20 to 25 minutes, until the sauce bubbles around the edges and the top turns golden.
Scatter the crispy shallots generously over the hot casserole. Let it rest 10 minutes before serving so the sauce sets slightly.

Author: Kozina Recipes
Prep Time: 20 minutes
Cook Time: 40 minutes