Place the haddock, salmon, prawns, onion, bay leaf and peppercorns in a large saucepan, pour over the milk and cream and bring to the boil, then remove from the heat and leave to stand for 5 minutes.
Remove the fish and prawns from the liquid and flake the fish into large pieces in a bowl. Set aside. Strain the liquid and discard the flavourings.
Melt the butter in a saucepan over a medium heat, then add the flour and stir well to combine. Cook for 2 minutes, then remove from the heat. Gradually add the strained liquid to the pan, stirring continuously. Return the pan to the heat and continue stirring until the sauce comes to the boil. Simmer for a few minutes, then season with salt and pepper. Pour the sauce into the bowl with the fish and gently stir so that the sauce coats all the fish. Pour into an ovenproof dish.
Cook the whole potatoes in lightly salted boiling water for about 10 minutes until they are almost cooked through. Remove from the water and slice into thin rounds about 3 mm (⅛ inch) thick. Arrange the potato slices on top of the fish and then dot with the butter. Sprinkle the Parmesan over the top and place in a preheated oven, 180°C (350°F), Gas Mark 4, for 25 minutes until the potatoes are golden brown.