Heat the oven to 375°F and grease a 9-by-13-inch dish. Cook the rotini 2 minutes short of done and drain.
Brown the beef in a large skillet and drain. Add the pepperoni, bell pepper, mushrooms and onion and cook for 8 minutes until the vegetables are tender and dry.
Stir in garlic for 30 seconds, then crushed tomatoes, oregano, salt and pepper and simmer for 6 minutes.
Fold in the rotini and 2 cups mozzarella. Spread in the dish and top with the remaining cheese.
Bake for 25 to 30 minutes until bubbling and browned in spots. Rest 10 minutes.