Preheat oven to 500°.
Cut bread loaf in half horizontally. Place both halves of bread, cut side up, on a large baking sheet. Spread 1/4 cup pizza sauce over each half of bread. Divide onion rings and mushrooms evenly between bread halves. Crumble beef into 1/2-inch pieces, and divide beef evenly between bread halves. Sprinkle Italian seasoning, garlic powder, and red pepper evenly over each pizza, and top each with 3/4 cup cheese.
Bake at 500° for 9 minutes or until beef is done and cheese melts. Cut each half into 3 equal pieces.