Cut the celeriac in half through the top and slice each half into 1/8- inch thick slices. Cut the slices, stacked 2 or 3 together into 1/8 inch wide julienne. In a mixing bowl, toss the julienne with the lemon juice. In another mixing bowl combine the mayonnaise with the mustard and season to taste with salt and pepper. Stir this sauce with the celeriac and parsley. Taste and adjust seasoning.