In a large bowl, toss together the shredded cabbage, carrots, and chopped onion.
In a separate bowl, whisk together the mayonnaise, mustard, sugar, apple cider vinegar, pepper, cayenne pepper, and salt until smooth.
Pour the dressing over the vegetables.
Toss until everything is evenly coated.
Cover and refrigerate for at least 2 hours before serving.
Toss again before serving.