In a large bowl, add mayonnaise, vinegar, dijon mustard, granulated sugar, celery seed, salt, and pepper. Whisk until smooth.
Pat your coleslaw mix dry with paper towels to remove excess moisture. To the dressing bowl, add shredded coleslaw mix and red onions. Toss until coated.
Refrigerate for at least 20 minutes (up to 2 hours) to allow flavors to meld and cabbage to soften slightly. Toss once more before serving if needed.
