In a skillet, cook sausage and onion until sausage is not longer pink and onion is tender; drain and cool slightly. Core apples; cut in half horizontally. Place in an ungreased 13-in. x 9-in. baking pan. In a bowl, combine egg and brown sugar; stir in sausage mixture. Spoon into the apples. Cover and bake at 350° for 15 minutes. Uncover and bake 15 minutes more or until apples are tender. Serve with syrup.