This is a favorite of my teenaged son and his friends. They've been known to take seconds and even third helpings of this. Even the pickiest of eaters like this casserole--one of my son's friends who is extremely picky asked for the recipe!
Spray deep 10x15 pan with non-stick spray. I use my lasagna pan. Cook the ground beef with onion and garlic and seasoned salt and pepper until meat is browned and onion is tender. Drain grease and mix in the Parmesan cheese.
Chop half of the pepperoni into small pieces and add to beef mixture. Leave the remaining pepperoni in slices. Stir in drained mushrooms and pasta sauce and simmer for 20-25 minutes on top of stove.
Preheat oven to 350. Place cooked rigatoni pasta in the bottom of greased baking dish. Sprinkle 2 cups mozzarella cheese over pasta. Layer the pepperoni slices evenly over the cheese. Top with all of the pasta sauce. Bake uncovered for 30 minutes.
Remove from oven and sprinkle remaining 2 cups of mozzarella cheese on top. Return to oven for 10 minutes until cheese is melted. Remove from oven and let stand 15 minutes before serving.