This is a delicious first course or can be served as a light lunch, perhaps with chunks of bread. The mustardy mayonnaise enhances the crunchy celeriac’s unique taste.
Using a large knife, cut each celeriac in half and peel away the skin, cutting about 3 mm (⅛ inch) deep under the skin (the skin is very fibrous so it is important to cut off enough). Coarsely grate the celeriac and put the flesh in a bowl. Season with salt and pepper and toss in the lemon juice. Cover with plastic wrap and set aside for 30–60 minutes.