Prep the vegetables:
Thinly slice the cucumber and celery, and chop the parsley. Combine them with the coleslaw mix in a large bowl.Make the dressing: In a separate small bowl, whisk together the mayonnaise, sour cream, white vinegar, cracked black pepper, salt, and onion powder.Toss and chill: Pour the dressing over the vegetables and toss until everything is evenly coated. Chill in the refrigerator for at least 30 minutes before serving to let the flavors blend