01 - Combine green cabbage, red cabbage, carrot, and green onions in a large mixing bowl.
02 - Whisk together mayonnaise, apple cider vinegar, Dijon mustard, sugar, celery seed, salt, and pepper in a separate small bowl until smooth and well combined.
03 - Pour the dressing over the vegetable mixture and toss thoroughly until all ingredients are evenly coated.
04 - Taste the coleslaw and adjust salt, pepper, or vinegar as needed to achieve desired flavor balance.
05 - Cover the bowl and refrigerate for at least 30 minutes before serving to allow flavors to develop and meld together.