In a large bowl, combine the onion, Parmesan, bread crumbs, egg, ketchup, garlic, salt, and pepper. Add the beef and veal. Using your hands, combine all ingredients gently but thoroughly. Shape the meat mixture into 16 (1 1/2-inch-diameter - golf ball size) meatballs and place on the prepared baking sheet. Bake the meatballs for 15 minutes until cooked through.