Poach salmon in water to cover. Drain and save stock.
Sauté mushrooms in butter and add lemon juice just before removing from heat. Drain and add cooking liquid to fish stock.
Mix fish, mushrooms and celery together. Chill while you prepare the dressing.
Melt the butter over low heat. Stir in the flour a little at a time until smooth. Add the stock gradually, stirring after each addition. Remove from heat and let cool somewhat. Add sugar and vinegar and mix well. Add egg yolks and stir until the mixture is smooth. Add the white wine, salt, pepper and a little minced parsley, mixing well.
Fold dressing gently into the chilled fish mixture.
Serve on a bed of lettuce leaves.