Gather a large mixing bowl and combine the coleslaw mix, sliced Granny Smith apple, diced celery, and finely sliced green onions.

In a medium saucepan, combine apple cider vinegar, granulated sugar, olive oil, Dijon mustard, and red pepper flakes. Heat over medium heat for 3-5 minutes until the sugar dissolves.

Remove the saucepan from heat, whisk in the mayonnaise until creamy. Pour the warm dressing over the salad mixture and toss thoroughly.

Cover the bowl and refrigerate for at least 1 hour to allow flavors to meld.