Shred the green cabbage, red cabbage (if using), and carrots into thin strips. You can do this by hand with a sharp knife, or use a food processor with a shredding attachment.
In a small bowl, whisk together the mayonnaise, vinegar, mustard, sugar, salt, and pepper until smooth and creamy. Add celery seeds if you’d like a slightly different taste profile.
In a large mixing bowl, combine the shredded vegetables. Pour the dressing over the vegetables and toss everything together until the cabbage and carrots are fully coated.
For the best flavor, let the coleslaw sit in the fridge for at least an hour. This allows the flavors to meld together and the cabbage to soften slightly.
