Place the lemon juice and salt in a bowl. Coarsely grate the celeriac. Immediately toss in the bowl with the salt and lemon juice. Leave for 30 minutes.
Meanwhile, warm another bowl in hot water and dry it. Add the mustard and egg yolk and whisk together to blend. Whisk in the oil, drop by drop. After about a quarter of the oil has been added, the rest can be poured in more quickly. Thi