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1
Heat the oven to 230°C. Season the duck and roast for 18–20 minutes. The dish lives or dies here: remove the bird when the breast registers 52–55°C internal temperature — the flesh must remain deeply, uncompromisingly pink. Any further cooking renders the blood-rich tissues unsuitable for pressing and destroys the sauce.
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2
Rest the duck briefly. Carve the breasts tableside: slice them thin and hold warm, covered, off direct heat. Remove the legs and reserve them for grilling. The breast slices should show an almost purple-red interior throughout.
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3
Place the carcass — with all remaining flesh, blood-saturated bones, and juices — into the duck press. Apply sustained, forceful pressure. The extracted juice flows thick and dark red, carrying concentrated blood and marrow. Continue pressing until the flow ceases entirely.
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4
In a chafing dish over gentle, controlled heat, combine the press juice with the reduced red wine and the cognac. Whisk in the foie gras mousse in small additions until fully incorporated, then mount with the cold butter piece by piece. Add the lemon juice. Throughout this step, the temperature must remain strictly below 65°C — blood proteins coagulate at 70°C and the sauce will become grainy and broken beyond recovery. The finished sauce should be the consistency of double cream and an almost black-red in color.
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5
Nap the sliced breast immediately with the sauce and serve at once. Delay causes the sauce to cool and the proteins to tighten.
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6
Grill the reserved legs over direct heat until the skin is blistered and the flesh is cooked through. Serve as a second course alongside a simple dressed salad. The legs, well colored and aromatic, form a deliberate contrast to the rare, sauced breast.