Preheat olive oil in large skillet over medium heat. Combine onion, garlic, salt, pepper and ground beef together in a large bowl. Form into small meatballs and place into hot pan. Cook about 2 minutes per side until browned. Reduce heat and stir in pizza sauce. Gently coat meatballs in sauce, adding in water where necessary to thin sauce. Cover to keep warm until meatballs are thoroughly cooked.
To assemble sandwiches, preheat broiler to high. Open hoagie rolls and place 4-5 meatballs and sauce in each. Top with 1/4 cup mozzarella cheese and 4 pepperoni slices. Broil until cheese is melted and pepperoni is sizzling. Serve hot.