Peel then shred a medium-sized (450g) celeriac. The shreds should not be too fine, nor should they be thicker than a matchstick.
Toss them immediately in the juice of half a lemon.
Mix together 4 heaped tbsp of good mayonnaise, 2 tbsp of smooth Dijon mustard, 2 tbsp of double cream or crème fraîche and 2 tbsp of chopped parsley.
Season with salt and black pepper, then fold into the shredded celeriac.
Set aside for 30 minutes then serve with thin slices of ham.
