Whisk together the mayonnaise, buttermilk, lemon juice, cider vinegar, vinegar, vegetable oil and sugar in a medium bowl. Add the salt, pepper, celery seed, onion powder and granulated garlic. Whisk until well combined.
Slowly start adding the dressing to the slaw, stirring as you go. Stop when the slaw is the desired consistency. I used all of the dressing, you might not. Reserve any unused dressing for later use.
Refrigerate slaw for at least 1 hour before serving.