Preheat oven to 400 deg. F
In a large bowl combine eggs, oats, ketchup, and onion. Mix well. Crumble beef over mixture and mix well. Coat 8 muffin tin cups with cooking spray. Fill each 3/4 full with the beef mixture. Sprinkle ketchup over each. Bake in 400 degree oven for 20-25 minutes. Sprinkle cheese on top and bake 5 minutes longer. Let stand 5 minutes before removing from muffin cups.