Whisk the sugar and 2 teaspoons salt with the vinegar in a large bowl until dissolved. Add the celery seeds, mustard seeds, 1/2 teaspoon pepper, the bell peppers, celery, scallions and cabbage, and toss to combine. Refrigerate at least 1 hour to allow the flavors to develop.
Add the mustard and mayonnaise to the slaw and toss to coat. Refrigerate until ready to serve.
Photograph by Con Poulos
Recipe courtesy Food Network Magazine