How to Make It
Preheat the oven to 425°. In a 3-quart baking dish, toss the mushrooms with the fries and olive oil. Bake until the fries are crispy, about 45 minutes. Remove from the oven and turn on the broiler.
Meanwhile, in a small saucepan, melt the butter. Add the flour and cook, whisking, until golden, 5 minutes. Whisk in the milk and cook until thickened, about 3 minutes. Season with salt and pepper. Spoon the gravy over the mushrooms and fries and top with the cheese. Broil 6 inches from the heat until golden, about 5 minutes.