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

In a saucepan over medium heat, whisk together the apple cider vinegar, granulated white sugar, olive oil, Dijon mustard, and red pepper flakes. Bring this mixture to a simmer to dissolve the sugar.

Once the dressing has simmered and sugar is dissolved, whisk in the mayonnaise until smooth. Pour this warm dressing over your cabbage mixture and toss everything together gently.

Cover the bowl with plastic wrap and place it in the refrigerator. Allow the slaw to chill for at least one hour.