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.