Cut the fish into pieces and arrange on a round dish.
Cover and microwave on HIGH for for 5 minutes.
Remove lid and leave to cool.
Put the butter in a bowl and soften in the microwave oven for 30 seconds on HIGH.
When the fish is cooked, remove the bones and skin and puree the flesh in a food processor.
Break 1 egg and separate the white from the yolk.
Add the melted butter, milk, egg yolk and 2 whole eggs to the fish puree, puree once more.
Add salt and pepper to taste.
Pour the mixture into a bowl.
Add a pinch of salt to the remaining egg white and whisk to form soft peaks.
Fold gently into the fish mixture.
Pour the mixture into a souffle dish, 7 inches (18 cm) in diameter.
Put the souffle dish into the microwave oven and microwave on HIGH for 10 minutes.
Leave to stand for 5 minutes.
To prepare the sauce, mix together the sour cream (creme fraiche), cognac and tomato juice in a bowl.
Heat in the micro wave oven on HIGH for 2 minutes.
Taste and adjust seasoning.
Rinse the tarragon and pat dry on kitchen paper.
Remove the leaves, chop and add to the sauce.
Turn out the gateau on to a serving dish.
Garnish with the shrimp (prawns).
Pour the sauce into a sauceboat and serve alongside.
This dish can also be prepared with cooked sea trout.
Allow for 1 1/4 lb (500 g) of fish including the bones and skin.