In a large bowl, whisk together mayonnaise, yogurt, sugar, vinegar, celery seed, and mustard (if using) until smooth. Season with salt and pepper, to taste.
Add shredded cabbage/carrot mixture. Mix well until dressing coats the cabbage.
Cover bowl tightly and refrigerate for at least one hour. Stir well before serving.
Serve garnished with freshly cracked black pepper or chopped fresh parsley, if desired.
