Place the fish in a pan with enough milk to cover and cook till tender. Drain thoroughly and chop finely.
Heat the oven to 400ºF.
Prepare the "bechamel" sauce by melting the butter in a pan over a medium heat. Add the flour working it in thoroughly with a wooden spoon. Remove the pan from the heat, add the milk and whisk until smooth. Season well with salt and pepper and grated nutmeg. Return the pan to the heat to cook for about 10 mins stirring frequently.
Remove the pan from the heat, beat in the yolks one at a time. Stir in the cheese until it dissolved then add the fish and peeled shrimps (or tomato purée). Leave to cool.
Beat the egg whites to a stiff peak 35 mins before serving. Place a third on top of the bechamel sauce and fold in using a wooden spatula. Add the rest of the egg whites in the same way, do not stir or beat.
Pour into the soufflé dish, previously greased, to about three quarters full.
Place in the oven immediately and cook for 20 mins then increase the heat to 450ºF for 10 mins and serve without delay.