In a medium bowl, whisk together the mayonnaise, apple cider vinegar, Dijon mustard, sugar, celery seeds, sea salt, and freshly ground black pepper until smooth.
In a large mixing bowl, combine the shredded green cabbage, shredded red cabbage, julienned carrots, and thinly sliced red onion.
Pour the dressing over the vegetable mixture and toss well to coat evenly.
Cover and chill in the refrigerator for at least 1 hour before serving to allow the flavors to meld and the cabbage to soften slightly.
Notes
The coleslaw can be made up to 2 days in advance. Store in an airtight container in the refrigerator. Stir before serving and adjust seasoning with an extra pinch of salt or a squeeze of lemon if needed.