For many, the word casserole brings to mind this Thanksgiving classic, most often made with what we consider to be the Holy Trinity of Casserole Ingredients: condensed cream of mushroom soup, frozen or canned green beans, and packaged french fried onions. While the classic version is comfort food for some of us, others just can’t stomach it. So we freshened up the recipe a bit—which means no cans of anything.
Heat the oven to 400°F. Bring a large pot of salted water to a boil. Add green beans and cook until bright green and just tender, about 6 minutes; drain in a colander and rinse under cold water until cold. Set aside.
Heat olive oil in a large frying pan over medium-high heat. When it shimmers, add mushrooms and season with salt and freshly ground black pepper. Sauté, stirring occasionally, until mushrooms are brown on the edges, about 4 minutes. Add garlic and cook 1 minute more.
Combine green beans, mushroom mixture, béchamel sauce, and thyme in a large bowl and mix thoroughly. Taste and adjust seasoning if necessary. Transfer to a 2-1/2-quart baking dish and set aside.
Dust sliced shallots in flour and shake off excess. Set aside.
Pour vegetable oil into a 10-inch frying pan (the oil should be 1 inch deep). Heat over medium-high heat to 350°F, about 4 minutes.
Fry shallots in batches, until light golden brown on the edges, about 2 minutes. Remove to a paper-towel-lined plate and season with salt and freshly ground black pepper.
Top casserole with fried shallots and bake until shallots are golden and crispy and casserole is bubbly and heated through, about 12 to 15 minutes.