1. Combine sausage, egg, breadcrumbs, apple and cheese in a large mixing bowl; stir well. Shape into 4 dozen (1-inch) balls. In a large skillet over medium-high heat, cook in batches of 12 or 24, turning frequently until lightly browned, about 10 minutes. Drain if desired, then return to skillet.
2. In a small bowl, combine mustard, tomato sauce, maple syrup and vinegar. Pour over sausage balls. Cover and simmer for 30 minutes. Transfer to chafing dish or slow cooker; serve hot with toothpicks.