To make the dressing, combine the mayonnaise, sugar, mustard, vinegar, celery seed, salt, and ground peppers in a small bowl. Whisk until the dressing is smooth. Stir in the bell pepper and onion. Refrigerate the dressing until ready to use.
Just before serving, empty the coleslaw mix into a large bowl. Add the dressing a little at a time, mixing until nicely coated but not overdressed.