How to Make It
Preheat oven to 425°F with rack in bottom position. Heat oil in a large skillet over high. Add mushrooms and onion; cook, stirring often, until mushrooms are browned and softened, 7 to 8 minutes. Stir in marinara sauce and 4 ounces of the pepperoni (about 56 slices). Remove from heat.
Coat a deep 13- x 9-inch casserole dish with cooking spray. Arrange garlic bread, lying flat with cut sides up, in casserole dish. Top evenly with mushroom-pepperoni mixture. Sprinkle with mozzarella. Arrange remaining 2 ounces pepperoni (about 28 slices) over mozzarella. Sprinkle with Parmesan. Bake in preheated oven until cheese is bubbly and lightly browned, 12 to 15 minutes. Let stand 5 minutes. If desired, sprinkle with crushed red pepper.