First make the dressing by mixing the lemon juice, mayo, crème fraîche, mustard and black pepper to taste in a large bowl. Set aside.
Using a sharp knife, top and tail the celeriac, then stand it on one end and, following its curve, cut downward to remove the skin and nobbly bits.
Halve the celeriac and slice each half very thinly (use a mandoline, if you have one). Then, taking