Step 1

Heat the oven to 325ºF. Arrange a rack in the middle of the oven and heat the oven to 325°F.

Step 2

Prepre the vegetables. Thaw and drain any excess liquid from 1 1/2 pounds frozen green beans. Thinly slice 12 ounces white button or cremini mushrooms. Mince 2 garlic cloves.

Step 3

Cook the mushrooms. Melt 4 tablespoons unsalted 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.

Step 4

Make the sauce. Sprinkle 1/4 cup all-purpose flour evenly over the mushroom mixture and stir to combine. Cook until the flour is no longer dry, 1 to 2 minutes. Add 2 cups whole milk, 1 cup low-sodium chicken or vegetable broth, 1 teaspoon tamari or soy sauce, 1/2 teaspoon black pepper, and 1/2 teaspoon grated nutmeg and bring to a simmer. Simmer, stirring occasionally, until the sauce is thickened, 3 to 5 minutes.

Step 5

Add some of the fried onions and green beans. Remove the sauce from the heat. Add the green beans and 1/2 (6-ounce) 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.

Step 6

Top and bake. Sprinkle with the remaining 1 1/2 (6-ounce) cans fried onions. Bake until golden brown and bubbling, 12 to 15 minutes.

Step 7

Cool before serving. Let cool for 10 minutes before serving. The sauce will thicken as it cools.