What You’ll Need
2 pounds green beans
2 tbsp. olive oil, divided
1 pound cremini mushrooms, sliced, divided
6 tbsp. unsalted butter, divided
4 large sprigs thyme
2 tbsp. all-purpose flour
1¼ cups whole milk
1 cup heavy cream
4 garlic cloves, finely grated
½ cup grated Parmesan
Freshly ground black pepper
¾ cup French’s fried onions
1. Preheat oven to 375°. Working in batches, cook green beans in large pot of boiling salted water until bright green and halfway to tenderness, about 3 minutes per batch. Drain and let cool, then trim stem ends.
2. Heat 1 Tbsp. oil in a large skillet over medium-high. Without stirring, cook half of mushrooms until golden brown underneath, about 2 minutes. Continue to cook, tossing occasionally, until browned on both sides, then add 2 Tbsp. butter and 2 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 then transfer to a plate. Repeat with 2 Tbsp. butter, and remaining salt, oil, mushrooms, and thyme.
3. Melt remaining 2 Tbsp. butter in a large saucepan over medium-low. Whisk in flour and cook, whisking occasionally, until roux is golden brown and smells nutty, about 4 minutes. Then, whisk in milk and cream. Bring to a simmer, whisking often, until sauce is thick and bubbling, about 5 minutes. Remove from heat and whisk in garlic and Parmesan; season with salt and pepper.
4. Arrange green beans and mushrooms in a 2-qt. baking dish and cover with sauce. Cover tightly with foil and 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 then continue to bake just until onions are slightly darker and fragrant, about 3 minutes.
5. Let sit 10 minutes to set before serving