Note: In lieu of fresh trout, 1/2 lb cooked and flaked fish may be used.
Serves 6 as an appetizer, or 3 as a light lunch.
If using fresh trout, poach them in the oven at 350 F (180 C) gas mark 4 for 30 minutes, then cool, skin, bone and flake - there should be about 8 ounces of cooked flesh.
Separate the egg and use the yolk to make mayonnaise: season it with salt and pepper, gradually beat in the oil, then 1 teaspoon or so of vinegar to thin and flavour the sauce.
Put the prepared fish into a food processor. Add the mustard, yoghurt and 1 heaped teaspoon each fresh chopped tarragon and parsley, and whizz to a 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 1/4 teaspoon each salt and caster sugar and 1/2 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.