Slice the potatoes into thick slices, and cook in a large saucepan of boiling salted water for 5 minutes then drain well.
Put the oil and leeks in a frying pan and cook over a low heat for 2-3 minutes until soft, add the mushrooms and cook for a further 2 minutes.
Place the fish in a saucepan and cover with a little milk, bring to a simmer and poach for 5 minutes until tender, drain but reserve the poaching milk!
Use a little of the butter to grease the baking dish. Flake the fish and place in the dish with the leeks, mushrooms and prawns/mussels and scatter over the parsley.
Preheatthe oven to 200 degrees C / 400 degrees F / Gas 6.
In the same saucepan, make a white sauce by melting the remaining butter and adding in the flour, stir well and cook over a low heat for 2-3 minutes. Remove from the heat and gradually stir in the remaining milk, beating well after each addition.
When all the milk has been added and the sauce is smooth, return the saucepan to the heat, stirring continously until thickened. Cook for a further 2-3 minutes then add 100g of the cheese and season to taste.
Pour half the sauce over the fish, arrange the potato slices on top and cover with the remaining sauce.
Sprinkle over the remaining cheese and cook on a baking sheet in the centre of a preheated oven for 45 minutes until golden brown.