Preheat oven to 400°. In a large skillet, cook beef over medium heat until no longer pink; drain. Stir in pizza sauce and mushrooms; set aside. , Cut bread in half lengthwise, then into eight pieces. Spread meat sauce on bread; place on a greased baking sheet. Sprinkle with mozzarella. Bake, uncovered, 10 minutes or until cheese melted and bubbly.