Slice the celeriac very thinly, using the width of a match as a rough guide, then lay a manageable pile of slices flat and slice again into match widths. At this point a mandolin is very useful, but if you don’t have one, do not fear, you can easily achieve matchstick strips of celeriac with a knife. As you go squeeze lemon juice over your growing mound of celeriac strands to prevent them going brown when finished. Fold the Dijon mustard and crème fraîche gently together—don’t beat, as the cream will lose its structure. Season to taste and mix this through the celeriac.