1Use a sharp knife to carefully peel the celeriac and remove the knobbly outer surface. Put 1 litre (34 fl oz/ 4 cups) cold water and half the lemon juice in a large bowl. Cut the celeriac into thin julienne strips and put them immediately into the lemon water to prevent discoloration. Soak for up to 1 hour.
2Bring a large saucepan of water to the boil and add the remaining lemon juice. Drain the celeriac and add to the boiling water. After 1 minute, drain and cool under cold running water. Pat dry with paper towels.
3To make the remoulade, whisk the egg yolks, vinegar and mustard together in a bowl. Add the oil, drop by drop from the tip of a teaspoon, whisking constantly until the mixture begins to thicken, then add the remaining oil in a very thin stream. Season and, if necessary, thin with a little warm water.
