How to Make It
Split loaf in half horizontally. Scoop out soft centers, making top and bottom bread shells about 1/2 inch thick. Reserve centers for another use. Set bread shells, cupped side up, on a 12- by 15-inch baking sheet. Bake in a 400° oven until bread is toasted, about 8 minutes.
Line bread shell cavities with mozzarella, overlapping slices as needed, and draping onto the rim of the bread.
In a 10- to 12-inch nonstick frying pan over high heat, frequently stir meat, mushrooms, and oregano until meat is browned and crumbly, 7 to 8 minutes. Add half the marinara sauce, half the olives, 3/4 cup onions, and salt and pepper to taste.
Fill bread shells equally with meat mixture. Top meat filling with remaining marinara sauce, olives, and onions. Sprinkle with parmesan.
Bake in a 400° oven until hot throughout, 10 to 15 minutes. Cut crosswise into 2- to 3-inch sections.