Prep the veggies: Finely shred the cabbages and carrot (or radishes).
Aim for thin, even strands so the dressing coats everything well.
Make the dressing: In a small bowl or jar, whisk together mayonnaise, vinegar, sweetener, Dijon, celery seed, salt, pepper, and sour cream if using. Taste and adjust sweetness, salt, and acidity. Add lemon juice if you want extra zing.
Toss to coat: Add the shredded cabbages, carrot (or radishes), and green onions to a large bowl.
Pour the dressing over and toss until the veggies are evenly coated.
Rest for flavor: Let the slaw sit for at least 15–20 minutes in the fridge. This softens the cabbage slightly and blends the flavors.
Final seasoning: Give it a quick toss and taste again. Add a pinch more salt, pepper, or sweetener if needed.
Serve: Garnish with a few extra green onions or a sprinkle of celery seed.
Serve chilled.