Heat 25g/1oz of the butter in a medium pan, add the shallot and cook gently for 3 minutes, until soft. Add the mussels and 150ml/5fl oz of water, then cover and cook over a high heat for 3-4 minutes, until the mussels have opened. Tip them into a colander set over a bowl to collect the cooking liquor. When they are cool enough to handle, remove the mussels from all but 8 of the nicest shells. Cover and set aside.
Pour all the mussel liquor except the last tablespoon or two (which will be gritty) into a 30cm/12in saute pan, bring to a simmer and then add the haddock, skin-side up. Cover and simmer gently for 3 minutes. Remove from the heat (leaving the lid in place) and set aside for about 4 minutes to continue cooking.
Meanwhile, melt another 25g/1oz of the butter in a large pan. Add the spinach and stir over a high heat until it has wilted. Cook, stirring briskly, until all the excess liquid has evaporated, then season to taste with some salt and pepper.
Divide the spinach between 4 warmed plates and put the haddock on top. Keep warm. Return the saute pan to the heat, add the remaining butter and boil rapidly for 3-4 minutes, until the liquor has reduced and emulsified into a sauce. Stir in the whisky and lemon juice and boil for 30 seconds. Add the chervil and mussels and stir for a few seconds, until they have heated through.
Spoon the mussels around the spinach and haddock, dividing the unshelled mussels equally between the plates, then pour over the sauce and serve.
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