Creamy Green Bean Casserole

Preheat oven to 375° F. Boil a large pot of salted water and add trimmed green beans. Cook for about 3 minutes, or until bright green. Drain and let cool.

Heat 2 Tbsp. olive oil in a large skillet over medium-high. Cook mushrooms until golden brown on both sides, about 5 minutes. Add 4 Tbsp. unsalted butter, garlic and thyme sprigs to pan. Cook, tossing occasionally, until butter browns and mushrooms are dark brown and very tender, about 4 minutes longer. Season with salt and transfer to a plate.

Melt remaining 2 Tbsp. butter in a large saucepan over medium-low. Whisk in 2 Tbsp. flour and cook, whisking occasionally, until roux is golden brown and smells mildly nutty, about 4 minutes. Whisk in 1¼ cups whole milk and 1 cup heavy cream. Increase heat and bring to a simmer, whisking often, and cook until béchamel sauce is thick and bubbling, about 5 minutes. Remove from heat and whisk in lemon zest and parmesan. Taste and season with salt and pepper.

Arrange green beans and mushrooms in a 2-qt. baking dish. Pour sauce over bean mixture and cover tightly with foil. Bake until sauce is bubbling, 25–30 minutes. Uncover and continue to bake until casserole is lightly browned on top and around the edges, 15–20 minutes longer. Top with fried onions and continue to bake just until onions are slightly darker and fragrant, about 3 minutes. Let sit 10 minutes to set before serving. Enjoy!