How to Make It
Combine sugar, water, vinegar, celery seeds, mustard seeds, and salt in a medium saucepan; bring to a boil over high, stirring occasionally. Boil 1 minute.
Combine cabbage, carrot, bell pepper, and onion in a large bowl. Pour hot vinegar mixture over slaw mixture; cover with plastic wrap, and chill at least 12 hours or up to 7 days. Sprinkle with scallions just before serving.