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1
POACHING METHOD — Combine the Muscadet, shallots, white peppercorns, bouquet garni, and enough cold water to submerge the fillets in a wide, shallow pan. Bring the court-bouillon to temperature over moderate heat. It is ready when a thermometer reads a steady 72–75 °C: the surface trembles but does not simmer.
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2
Slide the fillets into the court-bouillon and maintain the temperature strictly at 72–75 °C for 8–10 minutes, adjusting the heat as needed. The fish is done when the flesh has turned from translucent to barely opaque, with a faint translucency persisting at the very centre. Above 75 °C the sandre becomes dry and fibrous; that line must not be crossed.
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3
PAN-ROASTING METHOD (ALTERNATIVE) — Heat clarified butter in a heavy pan over medium-high heat until it shimmers. Lay the fillets skin-side down and press lightly to prevent curling. Cook for 4 minutes until the skin is deeply crisped and releases cleanly from the pan, then flip and baste with the foaming butter for a further 2 minutes.
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4
FOR THE BEURRE BLANC — Combine 200 ml Muscadet with 3 finely chopped shallots (or use the strained poaching court-bouillon for the poached version) in a small, heavy saucepan. Reduce over moderate heat until only 1–2 tablespoons of syrupy liquid remain. The reduction is correct when the bubbles slow and the liquid coats the back of a spoon in a thin, glistening film.
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5
Reduce the heat to its absolute minimum. Begin whisking in the cold butter one or two pieces at a time, incorporating each addition fully before adding the next. The emulsion builds gradually into a glossy, ivory, butter-cream sauce. One hand must remain on the exterior of the pan throughout: the warmth felt should be gentle, never hot. The sauce must hold between 58–63 °C — below that the butter solidifies; above it, the emulsion breaks into a greasy separation that cannot be recovered. This is where the dish lives or dies.
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6
When all 250 g of butter has been mounted and the sauce is glossy and pourable, remove from heat and finish with a few drops of lemon juice. Taste and season. The completed beurre blanc should be shimmering, ivory-gold, and hold its emulsified body without pooling or separating on the plate.
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7
Lift the poached fillets with a wide slotted spatula and drain briefly. Place each fillet on a warmed plate, nap generously with beurre blanc, and serve immediately alongside steamed potatoes or plain white rice. The sauce must coat the fish in a continuous, unbroken sheen.