Place the cabbage and carrots in a very large mixing bowl.
In a small bowl, whisk to combine the mayonnaise, apple cider vinegar, mustard if using, salt, and pepper.
Pour the dressing all over the cabbage, and toss well to combine.
Let the cole slaw rest in the fridge for at least 30 minutes before serving, then enjoy!
Here's a quick visual guide for How to Cut Cabbage. Storage: Keep the coleslaw in an airtight container in the fridge for up to 3 days. Technically it will be edible for more like 5 days, but the texture really starts to go downhill. Do not freeze.