Boil the potatoes in a large saucepan of salted water until tender; usually 20-25 minutes. Meanwhile, prepare the bechamel by studding the onion with the cloves. Place in a saucepan with the milk and bay leaf and bring to a simmer over a moderate heat. Set to one side to infuse a little.
Drain the potatoes and allow to cool a little before mashing with 50g (2oz) of the butter and some seasoning. Cover and set to one side.
Strain the milk into a jug, discarding the bay leaf but keeping the studded onion. Melt the remaining butter in a large, heavy-based saucepan over a medium heat, then sift in the flour and whisk until you have a smooth roux. Cook for 1-2 minutes, whisking occasionally, then add the milk in a slow, steady stream, whisking simultaneously, until you have a smooth, thickened sauce.
Simmer over a low heat for 8-10 minutes, stirring occasionally, the season to taste and whisk in the Cheddar and chopped parsley. Remove from the heat and discard the studded onion.
Meanwhile, pre-heat the oven to 200ºC/400ºF/Gas Mark 6.
Arrange the cod and haddock chunks in a 23cm (9in) squared ceramic baking dish. Pour over three-quarters of the bechamel sauce and mix well. Spread the mash on top, then pour the rest of the cheese sauce on top. Bake for 25-30 minutes until the top is coloured and bubbling. Remove from the oven and leave to stand for 5 minutes before serving.
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