Rinse the fish in cold water, pat dry and sprinkle with lemon juice. Boil water with salt, add fish, reduce heat and poach gently for 10 minutes. Remove from heat and leave the fish in the stock to cool for 30 minutes. Drain and break the fish into bite-size pieces. Skin and quarter three tomatoes and mix with the fish, together with the drained asparagus tips and walnut halves.

Dressing:- Mix the egg yolk with mustard and mayonnaise. Add the yogurt and season with salt, pepper and sugar. Blend in the sour cream. reserve 2 tbsp of the dressing and carefully fold the remainder into the fish salad. Marinate for 10 minutes. Line a flat dish with the lettuce leaves and pile the fish salad on top. Cover with the remaining dressing. Garnish with tomato and egg wedges, parsley and walnuts. Serve with fresh bread and butter. This is a lunch or dinner dish, alternatively it may be served with melba toast as a starter.