Add the coleslaw mix and diced onion to a medium-sized bowl.
Make the dressing by combining the mayonnaise, apple cider vinegar, mustard, powdered sugar, celery seed, salt, and pepper to a small bowl and whisk well to combine.
Pour the dressing over the coleslaw and onion mixture and toss well to combine.
If you have time, refrigerate the coleslaw for 30 minutes to 1 hour before serving to allow the flavors to meld together.
Serve on pulled pork sandwiches, pulled chicken BBQ sandwiches, on tacos, or as a side for BBQ.