Combine sugar, oil, and vinegar in a medium saucepan and bring to a boil. Reduce heat to medium. Stir to dissolve sugar.
Add celery seed, salt and pepper.
Add onion. Cook 1 minute. (Mixture should be gently simmering.)
Add bell pepper. Cook 1 more minute. Remove from heat.
Place cabbage mix in a large bowl. Pour hot sugar mixture over it. Stir to get it mixed in well. Let sit on counter until cabbage is wilted. Cover and refrigerate at least 4 hours or overnight.