Green bean casserole is a nostalgic part of the Thanksgiving table. This updated version is just as comforting — and more delicious: Green beans and sautéed mushrooms are bound with a gratin-style sauce, brightened with lemon juice, accented with nutmeg and loaded with Parmesan so that it’s bubbly with cheese. The traditional topping of fried onions, mixed with more Parmesan, is here. Or omit the fried onions and replace them with ½ cup regular or panko breadcrumbs mixed with 1/2 cup grated Parmesan.
This recipe is part of our Guide to Classic Thanksgiving, a collection of simple recipes for timeless holiday dishes to make this year and for years to come.
Bring a large pot of water to a boil and season with half a handful of kosher salt. Meanwhile, cut the beans crosswise in thirds. Add the green beans to the boiling water, and cook, stirring occasionally, until the beans are no longer snappy or crunchy but aren’t mushy — just past al dente — 5 to 6 minutes. To know when they’re tender, retrieve a bean from the water, place it on a cutting board and use a paring knife to cut the bean. If the knife goes through with little to no resistance, the beans are done. If you meet a little resistance, or the knife hits the cutting board from the pressure of having to cut into the bean, keep cooking the beans. Drain the beans in a colander and set aside until ready to use; reserve the pot.
While the beans cook, heat 2 tablespoons butter in a medium skillet. Add the mushrooms and ½ teaspoon salt and cook, stirring occasionally, until tender and golden brown, 8 to 10 minutes. Remove the skillet from the heat and let the mushrooms cool.
Return the pot used to boil the beans to medium heat and melt the remaining 4 tablespoons of butter. Add the shallots and garlic and cook, stirring occasionally, until just beginning to brown, 3 to 4 minutes. Switch to a whisk and sprinkle in the flour while whisking. Cook the flour to remove its raw taste, about 1 minute.
While whisking steadily, slowly pour the milk into the pot. Increase the heat to medium-high and continue cooking, stirring occasionally, until the sauce comes to a boil and thickens, 1 to 2 minutes more. Remove the pan from the heat and add the remaining 1½ teaspoons salt, the cooked beans and mushrooms, ½ cup Parmesan, the cream, lemon juice, black pepper and nutmeg. Stir to combine, then transfer to a 9-inch-square baking dish.
Heat the oven to 375 degrees. Sprinkle the fried shallots over the beans, followed by the remaining ¼ cup Parmesan. Bake the gratin until the top is golden brown all over and the sauce is bubbling in the center, about 40 minutes. Remove the dish from the oven and let cool for 5 minutes before serving.
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