01 - Place green cabbage, red cabbage, carrots, and red onion in a large mixing bowl.
02 - In a small bowl, whisk together mayonnaise, apple cider vinegar, Dijon mustard, sugar, celery seed, salt, and black pepper until thoroughly blended.
03 - Pour dressing over the vegetables and toss thoroughly until all ingredients are evenly coated.
04 - Taste and add additional salt or pepper if needed.
05 - Cover and refrigerate for at least 1 hour to allow flavors to meld before serving.