In a large bowl, combine the shredded cabbage and grated carrot.
In a separate small bowl, whisk together the mayonnaise, apple cider vinegar, Dijon mustard, erythritol (or sweetener of choice), celery seed, salt, and pepper until well combined.
Pour the dressing over the cabbage and carrot mixture. Toss well to coat the vegetables evenly.
Taste and adjust the seasoning if needed. You can add more salt, pepper, or sweetener according to your preference.
Cover the bowl and refrigerate for at least 1 hour to allow the flavors to meld together and the coleslaw to chill.
Serve the keto coleslaw as a side dish alongside your favorite keto-friendly main course.