Make the Dressing : In a large mixing bowl, whisk together the mayonnaise, Dijon mustard, whole grain mustard, apple cider vinegar, celery salt, kosher salt, and black pepper. Whisk until the mixture is smooth and creamy.

Combine the Ingredients : Add the shredded cabbage to the bowl with the dressing. Toss well to ensure the cabbage is evenly coated. Gently fold in the crumbled blue cheese, reserving a little for garnish if desired.

Chill and Serve : For the best flavor, cover the coleslaw and refrigerate for at least 30 minutes before serving. This allows the flavors to meld together. Serve chilled as a side dish or use it as a topping for sandwiches, burgers, or tacos.