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1
Place the beef in a deep, non-reactive container. Add the onion, carrot, celery, garlic, bay leaves, and cloves. Pour the Barolo over the meat; it should submerge it almost entirely. Cover and refrigerate for 24 hours. The wine will darken and carry the scent of the aromatics when the lid is lifted — that penetration is the signal the marinade has done its work.
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2
Lift the beef from the marinade and pat it thoroughly dry. Reserve the marinade and aromatics. Heat the oil in a heavy braising vessel (casserole or Dutch oven) over high heat until the oil shimmers. Lay in the beef and brown it aggressively on all sides without moving it prematurely — each face should be a deep mahogany, not grey. This is where the dish lives or dies: an insufficient crust will yield a sauce that is pale, thin, and without depth.
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3
Remove the browned beef and lower the heat. Add the reserved marinade vegetables to the pot and cook briefly until they soften slightly and take on some colour from the browned fond. Return the beef to the pot and pour over all the reserved marinade liquid. Bring to the barest simmer — 80–90 °C, with only the occasional lazy bubble breaking the surface. Cover tightly and maintain this temperature for 3–4 hours. Fassona Piemontese, being lean and firm, will require the longer end of this range. The meat is done when it yields completely to a spoon pressed into its surface.
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4
Remove the beef from the braising liquid and rest it in a covered dish for 20 minutes, allowing surface moisture to be reabsorbed. Strain the braising liquid, pressing the solids. Transfer the strained aromatics to a blender, add a ladleful of the braising liquid, and blend to a smooth purée; whisk this back into the strained liquid to build body in the sauce.
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5
Return the combined liquid to the heat and reduce over a moderate flame, skimming as needed, until it is thick enough to coat the back of a spoon. Remove from the heat and mount with the cold butter cubes, whisking steadily until the sauce is glossy and has a silky, tannic body that clings visibly to the whisk.
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6
Slice the rested beef across the grain into portions and arrange on warmed plates. Spoon the sauce generously over each slice; it should coat the meat in a rich, dark, luminous film rather than pooling thinly around it.