Regular wasabi powder can be used in place of pure wasabi powder. Adjust wasabi to taste depending on desired heat level.
In a large bowl, add the grated celery root, apple and onion and toss. Mix together the mayonnaise, sour cream, pure wasabi powder, vinegar, honey and green onions in a separate bowl. Season with salt and pepper to taste. Add dressing to the slaw 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.
Serve slaw atop burgers or other grilled fare to add an Asian twist to your meal. Dressing can also be used as a spread for burgers and sandwiches, a dip for wings or a topping for roast potatoes or vegetables.