In large bowl, placecabbage, bell pepper, and onions. Pour sugar over vegetables, but do not mix.
Place oil, vinegar, salt,ground mustard, and celery seed in a medium sized sauce pan. Heat mixture until warm. Do not boil.
Pour vinegar and oil mixture over cabbage mixture. Mix well.
Cover; refrigerate at least 3 hours, stirring several times, until chilled and flavors are blended.
Stir before serving;serve with slotted spoon.