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1
Slice the potatoes into thick slices, and cook in a large saucepan of boiling salted water for 5 minutes then drain well.
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2
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.
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3
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!
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4
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.
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5
Preheatthe oven to 200 degrees C / 400 degrees F / Gas 6.
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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.
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7
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.
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8
Pour half the sauce over the fish, arrange the potato slices on top and cover with the remaining sauce.
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9
Sprinkle over the remaining cheese and cook on a baking sheet in the centre of a preheated oven for 45 minutes until golden brown.
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10
Then serve!
