To peel the celeriac, lop off the root and opposite end with a knife. Stand the round root on the flat end, and using a large knife, cut downward, working around the outside to slice off the tough skin. Cut the root into julienne strips or cut in a food processor to matchstick style.
The root browns quickly, so as soon as it is grated, toss with lemon juice.
In a separate bowl, mix the yogurt, mustard, mayonnaise, parsley, salt and pepper. Pour over the root and mix. If the dressing is too thick, add more lemon juice.