Food Processor to the Rescue!
Wash and peel your celery root ball with a sharp veggie peeler. Cut the celery into long rectangles, which can fit into your food processor.
In a separate bowl, whisk together the yogurt, mayonnaise, mustard, oil, salt and pepper.
Using the coarse or medium grate setting on your food processor, grate all of the celery root.
Slowly add the yogurt mixture to the celery root, until the texture is the desired moistness.