First prepare the watercress sauce, Chop the watercress, reserving 4 sprigs for decoration, then put the crème fraîche and chopped watercress into a saucepan and stir together. Add lemon juice and salt and pepper to taste and set aside.
Put the grated potato into a bowl and season with salt and pepper. Divide the mixture into four cakes.
Heat the oil in a frying pan, add the potato cakes and cook over a moderate heat until browned all over and softened inside. Remove from the pan and set aside to keep warm.
Meanwhile, season the salmon fillets with salt and pepper. Line the grill rack with a piece of greased foil and arrange the fillets on top. Dot with butter and cook under a preheated grill for 4—5 minutes on each side until cooked through.
Heat the watercress sauce gently – it may be served hot or cold.
To serve, place a potato cake on each plate, put the salmon on top and pour the sauce over. Garnish with watercress sprigs and serve immediately.