1. First of all make the mash. Cook the potatoes in boiling salted water until soft. Then drain well and mash. Add the cream and butter, and season with salt and pepper, mixing really well so the mash is really smooth and creamy.

2. Prepare the sauce by heating the butter over a very low heat then stir in the parsley. Cook for about ten minutes. Then add the plain our and cook this mixture, without browning, for about 3 minutes, stirring a few times.

3. In another pan, bring the milk and cream to the boil. Pour this into the "roux" (the butter and our mixture) a little at a time, whisking or beating it. Bring the sauce to the boil and then let it cook over a very low heat for 5-6 minutes.

4. While the sauce is cooking, pre-heat the oven to 200C or Gas mark 6 and generously butter a deep ovenproof dish. Cut the raw fish into pieces and lay it in the dish, season lightly with salt and pepper and add a little lemon juice too.

5. Pour the hot sauce over the top and then cover by spooning over the potatoes
and sprinkle on the grated cheese. Bake for 35-40 minutes by which time the top should be crisp and brown and the fish perfectly cooked in its sauce.