Watercress Yoghurt Sauce

This is a perfect accompaniment for any cold fish, such as poached salmon, and is also good for cold mousses and terrines. I do tend to use a lot of yoghurt-based sauces for cold food, simply because yoghurt gives the sauce a light, fresh, clean taste. This sauce does not keep very well, as the lemon juice and the yoghurt turn the watercress grey. It is best made no more than half an hour before required and kept at room temperature – if it is kept in the refrigerator, it will become too co