Combine meats in a large bowl and add the egg. Sauté the onion in a pan over medium-high heat until browned. Add chicken stock to get fond off of the pan and cook until no liquid remains. Pour onto meat mixture. Add Parmesan cheese, parsley, and garlic. Mix well with your hands. Roll into 8 large balls and cook at 350ºF for 50 minutes (I like to bake them in the wells of a muffin tin). Remove from oven and discard fat.
Transfer meatballs to a glass baking dish. Pour over the jar of red sauce, top with more parmesan, and bake for another 30 minutes, or until cheese is melted and browned.