Combine first 3 ingredients in a bowl; stir well. Melt margarine in a small, heavy nonaluminum saucepan over low heat. Add cornstarch mixture to margarine, stirring well. Stir in lemon juice. Bring to a boil over medium heat, stirring constantly; cook 1 minute. Gradually stir about one-fourth of hot mixture into egg; add to remaining hot mixture, stirring constantly. Cook, stirring constantly, 3 minutes or until thickened.
Pour mixture into a small bowl; cover and chill. Serve over fresh fruit, reduced-fat scones, angel food cake, or low-fat ice cream. Store in an airtight container in the refrigerator.