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1
Combine the white wine, fish fumet, and minced shallots in a wide sauté pan. Bring to 75–80 °C — a bare tremble, not a boil. Add the scallops and corals and poach for 2–3 minutes. They are done when barely opaque at the centre; remove them immediately with a slotted spoon and slice the scallops into 5 mm rounds. Slice the corals. Reserve the poaching liquid.
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2
Raise the heat under the poaching liquid and reduce by two-thirds. Add the fish velouté and cream. Reduce over moderate heat, stirring, until the sauce coats the back of a spoon and flows slowly when the spoon is drawn across — nappant consistency. Finish off the heat with the butter and lemon juice.
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3
Fold the sliced scallops, corals, and mushrooms gently into the sauce. The mixture should be fluid enough to flow slightly when the shell is tilted.
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4
Prepare the duchess potato: beat the egg yolks and butter into the hot riced potato to a firm, smooth paste. Transfer to a piping bag fitted with a star nozzle and pipe a substantial border around the rim of four clean scallop shells. The border must be thick enough to hold its shape without cracking under the salamander.
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5
Fill the centre of each shell with the scallop mixture. Nap the surface with a spoonful of additional sauce, then dust with grated Gruyère and fine breadcrumbs.
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6
Gratinée under a salamander pre-heated to 280 °C for 2–3 minutes. The dish is ready when the duchess border is golden and the sauce at the edges shows active, small bubbles. Remove the instant the potato colours — the scallops below must not cook further.
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7
Set each shell on a plate lined with a folded napkin to prevent sliding. Serve without delay.