In a large bowl, toss together the cabbage and carrots. Set aside.
In a separate bowl or large measuring cup, whisk together the mayonnaise, sugar, milk, buttermilk, sour cream, vinegar, grated onion, lemon juice, salt, celery salt and pepper.
Pour the dressing over the cabbage mixture. Toss to coat.
Cover and refrigerate for at least 2 hours (or overnight) before serving.
Video
Notes
For the classic KFC coleslaw texture, use the shredding disc on a food processor to chop the the cabbage and carrots into a very fine dice. The food processor also makes the job very quick and easy!
Grate the onion very finely. I use a microplane for this task, which creates an onion pulp that blends smoothly into the dressing. You'll also want to include the juices from the grated onion for more flavor.
Chill the coleslaw for at least 2 hours (or overnight) before serving. This gives the vegetables a chance to marinate in the dressing for the best flavor and texture.
Just like KFC's, this coleslaw dressing is very thin and will accumulate at the bottom of the dish. That's normal! Just drain off any extra dressing or use a slotted spoon for serving.