In a small saucepan, saute onion in butter until tender. Stir in apple jelly and brown sugar; add sausage links. Cook, uncovered, over medium-low heat for 15-20 minutes or until thickened, stirring occasionally. Add apple.
Cover and cook over medium-low heat for 8-10 minutes or until apples are tender. Combine cornstarch and water until smooth; stir into saucepan. Bring to a boil; cook and stir for 1 minute or until thickened. Serve warm.