Combine the shredded cabbage and carrots in a large bowl.

Whisk together the mayonnaise, sour cream, onion, sugar, vinegar, mustard, celery salt, salt, and pepper in a medium bowl, and then add to the cabbage mixture.

Mix well to combine and taste for seasoning; add more salt, pepper, or sugar if desired.

Let the coleslaw rest for at least 30 minutes before serving to allow the flavors to meld.