In a large mixing bowl whisk together the mayonnaise, grated onion, apple cider vinegar, sugar and celery seeds. Add the shredded cabbage and carrot then toss to combine. Season with salt and pepper to taste. Let sit at least 1 hour before serving.
Recipe by Lemon & Mocha at http://www.lemonandmocha.com/2014/06/19/coleslaw/