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For the potatoes, boil them until tender. Drain them, put them back in the pan and dry them for 1-2 minutes over a low heat. Take off the heat and mash the potatoes with the butter, nutmeg, salt and pepper, then beat over a low heat until fluffy. Take the potatoes off the heat and beat in the egg yolks to thicken. Adjust the seasoning. Let the purée cool slightly, then spoon it into a large plastic sandwich bag with a 2cm/3⁄4in cut in a corner (or piping bag with a plain nozzle) and pipe a generous border of potato around each scallop shell.
Discard the white muscle from each scallop and cut any large scallops horizontally in half. Heat the fish stock and wine in a pan, add the scallops, cover and poach 1-2 minutes until the scallops just whiten (remember that they will cook further in the sauce). Transfer them to a bowl, reserving the stock separately.
Cook the mushrooms, covered, with the lemon juice, salt and pepper, in a thin film of water until tender, about 5-7 minutes. If any liquid remains, simmer uncovered until it has evaporated. Add the mushrooms to the scallops.
For the sauce, boil the reserved stock for 10 minutes until reduced to 450ml/16 fl oz. Fry the shallots in the butter until soft, 1-2 minutes. Stir in the flour and cook until it’s foaming. Whisk in the stock and bring to a boil, whisking, then simmer until the sauce thickens to coat a spoon, 3-5 minutes. Add the crème fraîche and any liquid released by the scallops, and simmer until the sauce reaches coating consistency. Season.
Heat the oven to 200C/Gas 6/fan 180C. Stir the scallops and mushrooms into the sauce and spoon into the shells. (Can be prepared ahead to this stage and kept in the fridge for 6-8 hours.) Sprinkle with the breadcrumbs and melted butter. Set the shells on a baking sheet, steadying them on a slice of bread or a bed of foil. Bake until browned and bubbling, 12-15 minutes. Serve very hot.