Place the haddock and milk in a saucepan; heat slowly and simmer gently for about 8 minutes or until the fish will just flake when tested with a fork. Remove the pan from the heat and leave the fish to cool in the milk. When the fish is cool enough to handle, remove it and flake the flesh, discarding the skin and any bones. Set the poaching milk aside to cool.
Preheat oven, with baking tray inside, to 190 C / Gas 5. Lightly grease a 1.7L (3 pint) souffle dish with butter. Mix grated Parmesan and breadcrumbs together and sprinkle over the bottom and sides of the dish to coat evenly. Shake out any excess crumb mixture and reserve for topping.
Mix cornflour with a little of the reserved poaching milk to make a smooth paste. Heat remaining milk in a small saucepan until almost boiling, then pour into the cornflour mixture, stirring constantly. Return to the pan and bring to the boil, stirring to make a thick sauce.
Pour the sauce into a large mixing bowl. Add the egg yolks, one by one, beating them thoroughly into the sauce. Stir in the flaked haddock, tomatoes, mustard, parsley, chives, salt and pepper to taste.
In a clean, dry mixing bowl, whisk the 4 egg whites until stiff enough to hold soft peaks. Fold a quarter of the whites into the sauce mixture to lighten it, then gently fold in the remaining whites.
Spoon the mixture into the prepared souffle dish and sprinkle the top with the reserved Parmesan and breadcrumb mixture. Set the dish on the preheated baking tray and cook for 35 minutes, or until well risen and golden brown. Serve at once.