perfectly smooth puree. Soak the gelatine (more may be necessary in hot weather) in 3 tablespoons cold water, let it soften and swell, then dissolve over low heat. Cool for a minute or two before adding the liquid gelatine to the fish puree: pour it on in a thin stream while the machine is running. Carefully and gradually beat the fish puree into the mayonnaise. Quickly whisk the egg whites to snowy peaks and fold them into the fish puree. Taste and adjust the seasoning. Divide the mixture between 6 ramekins or 3 small souffle dishes. Cover and chill for 2-3 hours until set to a soft cream. Seed the cucumber but do not peel. Chop it finely, sprinkle it with quarter teaspoon each salt and caster sugar and half teaspoon tarragon vinegar, and set aside for a couple of hours to draw out some of the cucumber juices. Drain and pat dry. Top each mousse immediately before serving with a small spoonful of the cucumber a good crunchy contrast to the soft textured mousse and garnish with a tarragon leaf. Oatcakes that have been gently warmed in the oven go well with this dish.