Trim outer leaves from cabbages. Remove core and finely shred green and red cabbage using a sharp knife, mandoline, or food processor. Peel and grate or julienne the carrots. Place shredded cabbage and carrots in a large bowl. If you want extra crispness, add 1 cup crushed ice over the shredded vegetables and let sit 5 minutes, then drain thoroughly and pat dry with paper towels.

Thinly slice the green onions and chop the parsley (if using). Add green onion and parsley to the bowl with the shredded vegetables.

Make Dressing

In a medium mixing bowl, whisk together mayonnaise, apple cider vinegar, buttermilk, and Dijon mustard until smooth.

Add sugar, honey, smoked paprika, crushed celery seed, kosher salt, and freshly ground black pepper. Whisk until sugar is dissolved and the dressing is homogeneous. Taste and adjust seasoning: add lemon juice for brightness if desired, or up to another 1 tsp of vinegar for tang.

Combine and Finish

Pour the dressing over the shredded cabbage, carrots, scallions, and parsley. Using two large spoons or salad tongs, toss thoroughly until all vegetables are evenly coated. Make sure to scrape bottom of bowl so dressing is distributed.

Cover the bowl and refrigerate at least 30 minutes, preferably 1–2 hours, to allow flavors to meld and cabbage to soften slightly. If serving immediately, let sit 10–15 minutes.

Before serving, taste and adjust salt, pepper, or a splash more vinegar or lemon juice if needed. Give a final toss and transfer to serving dish.

Store leftovers in an airtight container refrigerated up to 3 days. If the slaw becomes watery, drain excess liquid and toss again before serving.