Wash your cabbage thoroughly and remove the tough outer leaves. Cut into quarters, remove the core, and then slice into thin shreds or use a food processor with the shredding disc for perfectly uniform pieces.
In a large bowl, whisk together the mayonnaise, apple cider vinegar, sugar, mustard, celery seeds, salt, and pepper until smooth and well combined.
Add the shredded cabbage, grated carrot, and diced onion to your dressing. Using tongs or two large forks, gently toss until all vegetables are evenly coated.
Cover the bowl tightly with plastic wrap and refrigerate for at least 2 hours, preferably 4-6 hours or overnight to allow the flavors to meld.
Before serving, stir the coleslaw again to redistribute the dressing and serve chilled.