To prepare a vegetarian breakfast casserole with spinach and mushrooms, you'll need: 1 1/2 cups frozen spinach 1/2 cup frozen mushrooms 1 cup shredded Parmesan cheese 2 tablespoons melted butter 1 tablespoon kosher salt 1 teaspoon ground black pepper Instructions Preheat the oven to 350 degrees F. Line an 8-inch-square baking dish with foil. Pour the vegetable oil into the bottom of the dish. Spread the spinach mixture evenly over the foil-lined bottom. Sprinkle the mushrooms with salt and pepper. Bake in the preheated oven for 30 minutes or until the top is golden brown. Let the casserole cool for 10 minutes before slicing into squares.