Classic Green Bean Casserole from TheKitchn.com
https://www.thekitchn.com/how-to-make-classic-green-bean-casserole-237725

4 T unsalted butter
12 oz mushrooms, stems removed and chopped
2 cloves garlic, minced
1/4 c all-purpose flour
2 c whole milk
1 c low-sodium chicken or vegetable broth
1 t tamari or soy sauce
1/2 t freshly ground black pepper
1/2 t freshly grated nutmeg
2 (6-ounce) cans French's Fried Onions, divided
1 1/2 lbs frozen green beans, thawed and drained

Prepare the oven. Arrange a rack in the middle of the oven and heat to 325°F.
Cook the mushrooms. Melt the butter in a large, straight-sided skillet over medium-high heat. Add the mushrooms and cook until reduced by half, about 5 minutes. Add the garlic and cook until fragrant, about 1 minute.
Make the gravy. Sprinkle the flour evenly over the mushroom mixture and stir to combine. Cook until the flour is no longer dry, 1 to 2 minutes. Add the milk, chicken broth, tamari or soy sauce, pepper, and nutmeg, and bring to a simmer. Simmer until the sauce is thickened, 3 to 5 minutes.
Add some of the onions and green beans. Remove the sauce from the heat, add the green beans and 1/2 a can of the fried onions, and stir to combine. Transfer the green bean mixture to a 9x13-inch baking dish and spread into an even layer.
Top and bake. Sprinkle with the remaining 1 1/2 cans onions. Bake until golden and bubbling, 12 to 15 minutes.
Cool before serving. Let cool 10 minutes before serving. The sauce will thicken as it cools.

Recipe Notes
Make ahead: Assemble the casserole without the crisp onion topping and store tightly wrapped in the refrigerator for up to 3 days. Remove from the refrigerator while the oven preheats. Top with the fried onions and bake.
Storage: Store leftovers tightly wrapped in the refrigerator for up to 1 week.