Whisk together the lemon juice, mustard, and crème fraiche in a medium mixing bowl. Set aside.
Brush any excess dirt off of the celeriac. Cut off the bottom and top of the roots. Cut the roots into quarters and peel with a serrated peeler. Rinse in cool water if there is any remaining dirt or debris. Slice each quarter on a mandoline into 1-inch long matchstick pieces and transfer directly to the dressing. Add the salt and pepper, and stir well to combine. Cover and refrigerate for a minimum of 1 hour and up to 3 before serving.