The Best Coleslaw

In a large bowl, combine the shredded cabbage, grated carrot, and chopped red onion.
In a separate small bowl, whisk together the mayonnaise, apple cider vinegar, Dijon mustard, sugar, celery seed, salt, and pepper until smooth.
Pour the dressing over the cabbage mixture and toss until the vegetables are evenly coated.
Cover the coleslaw and refrigerate for at least 1 hour to allow the flavors to meld.
Before serving, toss again and garnish with fresh parsley if desired.