Prep the vegetables: If using a bagged broccoli slaw, pour it into a large bowl. Add finely shredded green cabbage and the thinly sliced red onion.
Make the dressing: In a separate bowl, whisk together mayonnaise, sour cream, apple cider vinegar, Dijon mustard, celery seed, and sweetener if using.
Season with salt and pepper.
Taste and adjust: Check the dressing. Add a pinch more salt, vinegar, or sweetener to balance creaminess and tang.
Combine: Pour the dressing over the slaw mix. Toss thoroughly with tongs until every strand is lightly coated.
Add herbs and crunch: Fold in chopped dill or parsley.
Sprinkle in nuts, seeds, or bacon if you like.
Rest briefly: Let the slaw sit for 10–15 minutes so the flavors meld and the vegetables soften slightly.
Serve: Give it a final toss and taste for seasoning before plating.