Prep the cabbage: Finely shred the green and red cabbage with a knife or mandoline.
Place in a large bowl and sprinkle with 1 teaspoon of kosher salt. Toss and let it sit for 10–15 minutes to draw out excess water.
Rinse and dry: After resting, rinse the cabbage briefly under cold water to remove the extra salt. Spin it dry in a salad spinner or pat dry very well with clean towels.
This step keeps your coleslaw from getting watery.
Add the veggies: Return the cabbage to the bowl. Add the grated carrots and red onion. Toss to combine.
Make the dressing: In a separate bowl, whisk together mayonnaise, sour cream, apple cider vinegar, Dijon mustard, sugar or honey, celery seed, 1/2 teaspoon kosher salt, and black pepper.
Taste and adjust sweetness or tang to your liking.
Combine: Pour the dressing over the cabbage mixture. Toss until everything is evenly coated. If it looks a little thick, add 1–2 teaspoons of water to loosen.
Chill: Cover and refrigerate for at least 30 minutes before serving.
This allows flavors to meld and the texture to set.
Finish and serve: Give it a quick toss and taste again. Add a pinch of salt, a squeeze of lemon, or a dash of vinegar if needed. Serve cold.