In a large mixing bowl combine ground beef, sausage, bread crumbs, milk, egg, parsley, cheese, Italian seasoning, salt, and pepper until well combined. Form mixture into balls, about 1-1/2 inches in diameter.
In a large skillet, cook the meatballs in hot olive oil over medium heat for 10 minutes or until browned, turning occasionally.
Bring marinara to boiling. Gently place meatballs into sauce. Stir gently to coat with sauce. Cover and cook for 30 minutes or until an instant-read thermometer inserted in the meatballs registers 160 degree F. Makes about 16 meatballs.