Combine sausage, egg, breadcrumbs, apple and cheese in large mixing bowl; stir well. Shape into 4 dozen 1-inch balls. In a large skillet over medium-high heat, cook meatballs in batches of 12 or 24, turning frequently, until lightly browned, about 10 minutes. Drain if desired, then return to skillet. In a small bowl, combine mustard, tomato sauce, maple syrup and vinegar. Pour over sausage balls. Cover and simmer for 30 minutes. Transfer to a chafing dish or slow cooker; serve hot with toothpicks. Note: Sausage balls may be prepared ahead and frozen uncooked; thaw in refrigerator before cooking as directed above.