Spray a 12-cup muffin tin with cooking spray, and preheat oven to 400 degrees
In a large mixing bowl, mix the beef, 2 tablespoons of marinara sauce, onions, Italian seasoning, garlic powder, salt, pepper, egg and Parmesan cheese together. Use hands to squish ingredients together and form into a large ball
Fill muffin cups evenly with meat mixture, and bake for 25 minutes
Brush marinara sauce over tops of muffins, and top each muffin with shredded mozzarella
Bake for additional 5 minutes, or until cheese is melted and internal temperature is at least 160 degrees, according to meat thermometer