Butter a large, fairly shallow gratinée or pie dish. Cut the fish up into pieces and lay them in the dish. Sprinkle with dill seeds, salt and pepper. Pour over the milk, cover the dish with foil and cook in a pre-heated oven at Gas 6 (400°F/200°C) for about 20 minutes. When you take the fish from the oven turn the temperature down to Gas 2 (300°F/150°C). Pour off the milk the fish has cooked in into a bowl and leave the fish in the dish. Put the chopped up ham amongst the fish. Melt about 2 oz butter in a saucepan. Remove from the heat and stir in the flour. Simmer gently for 2 minutes, stirring all the time. Gradually stir in the milk the fish was cooked in. Bring to the boil and simmer for 1 minute. Add the cheese and stir until melted and smooth. Cool slightly and then thoroughly stir in the egg yolks one by one. Check for seasoning. Whisk the egg whites until stiff and fold gently into the sauce with a metal spoon. Pour this mixture over the fish and ham and cook in the centre of the oven at Gas 2 (300°F/150°C) for about 50–60 minutes until risen and golden brown.