Skate with tartare sauce

Take a large piece of parchment paper and smear with the butter. Put the pieces of skate in the centre and season with salt. Fold the edges of the paper together. Steam over a pan of boiling water for 8-10 minutes, until cooked through.

For the tartare sauce: beat together the egg yolks, mustard, lemon juice and vinegar in a bowl. Add the oil very slowly, beating as you go, until it is all incorporated and you have a thick creamy mayonnaise. Stir in the remaining ingredients and season to taste.

To serve: put the cooked skate on a plate with a pile of watercress, lemon wedges and the tartare sauce in a bowl alongside.