Using a very sharp knife, finely chop the cabbage. Shred the carrots with a large grater. Combine in a large bowl and set aside. Alternatively, pour 2 bags of pre shredded cabbage and carrots in a large bowl. Set aside.

In a 2 cup liquid measuring cup, combine mayonnaise, yogurt, onion, sugar, vinegar, celery seed, dry mustard, salt and pepper. Whisk well to combine thoroughly. Taste, adding salt, pepper, mustard, and perhaps vinegar to taste.

Pour the dressing over the carrots and cabbage and mix. Flavors will develop more if you allow it to rest overnight