In a large bowl, add the celery root, apple, and onion and toss. Mix together the mayonnaise, sour cream, pure wasabi powder, and salt and pepper, to taste, vinegar, honey and green onions in a separate bowl. Add it to the celery root mixture and toss to combine. Cover with plastic wrap and refrigerate for at least 1 hour for the flavors to marry. Transfer to a serving bowl and serve.