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Smear the skin of the cod with the butter and season with sea salt. Heat 1 tbsp of the oil in a frying pan. When very hot add the cod, skin side down, and fry, without moving the fish, until the skin is crisp and golden. Invert on to a shallow ovenproof dish, ready to finish it off in the oven.
Put the cherry tomatoes in a separate shallow ovenproof dish and drizzle with another 1 tbsp oil.
To make the sauce, cook the shallots and wine in a small pan for about 2-3 minutes until the wine has almost gone. Tip into a food processor with the crème fraîche and watercress, season with salt and pepper and blitz. Add a squeeze of lemon juice to taste, return to the pan and set aside. Lightly whip the cream until it just holds its shape. (At this stage the cod, sauce and cream can be kept in the fridge, lightly covered with cling film, for up to 4 hours.)
About 20 minutes before you are ready to cook, preheat the oven to fan 170C/conventional 190C/gas 5. Roast the cod and tomatoes for 12-15 minutes until just cooked. Meanwhile, gently reheat the sauce, remove from the heat and fold in the cream. Taste and add more seasoning and lemon juice if necessary. Set the cod on warm dinner plates with the cherry tomatoes and spoonfuls of watercress sauce. Garnish with sprigs of parsley or chervil and serve with buttered new potatoes.