Make coleslaw dressing: Whisk together mayonnaise, sugar, vinegar, salt, and pepper in a large bowl. Refrigerate until ready to use.
Prepare cabbage: Cut cabbage head into quarters. Shred on the large holes of a box grater.
Prepare carrots and onion: Shred carrots and onion on box grater.
Mix coleslaw: Add cabbage, carrots, and onion to mayonnaise mixture. Stir well to combine. Refrigerate at least 30 minutes before serving.
