Flake the fish, discarding the skin and bones as you go, then put it on one side while you make a sauce by melting the butter in a saucepan and stirring in the flour.
Cook this for a minute or two before gradually adding the strained milk. Bring to the boil, stirring, and boil gently for 1 minute. Then remove the pan from the heat and beat in the egg yolks, 1 tablespoon of Parmesan cheese, the lemon juice, parsley, anchovy essence or paste, spring onions and the fish. Season well with salt and ½ teaspoon of cayenne pepper.
Next whisk the egg whites to the stiff-peak stage. Then fold them into the fish mixture, taste and season again if necessary. Now pour the mixture into a buttered 1.7 litre soufflé dish or deep baking dish, and sprinkle with the remaining Parmesan cheese. Transfer the soufflé to the oven, placing it on a baking sheet, and turn down the heat immediately to 190˚C/375˚F/gas mark 5.
Cook for 35-40 minutes, or until the soufflé is well risen and golden brown, then serve absolutely immediately.
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