Preheat oven to 375°. Coat metal muffin pan with nonstick spray. In medium mixing bowl, combine beef, onion, green pepper if desired, cereal, salt, pepper and egg. Set aside., In small bowl, stir together ketchup, brown sugar and mustard until smooth. Pour half of ketchup mixture into meat and mix until thoroughly combined. Form meat mixture into six balls and place in muffin cups. Spoon remaining ketchup mix over meatballs. Bake for 30 minutes. Remove from pan with spoon and serve.