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1
Lay the beef slices in a deep non-reactive vessel. Add the sliced onions, cloves, cinnamon stick, and juniper berries. Pour the wine over to submerge. Cover and refrigerate for 24 hours. The meat is ready when it has darkened throughout and carries the scent of wine and spice.
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2
Lift the beef from the marinade and pat it dry. Reserve the marinade — liquid, onions, and spices — in full. Melt the butter and lard together in a wide, heavy braising vessel over medium heat until the fat is unified and shimmering. Brown the beef slices in batches without crowding, turning once, until deeply coloured on both sides. Each batch is done when the meat releases cleanly from the pan and shows a mahogany crust.
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3
Return all browned beef to the vessel. Pour over the reserved marinade — liquid, onions, and spices — in full. Bring to a gentle simmer, then reduce the heat to the lowest possible setting. Cover and braise slowly, turning the meat occasionally, until the beef is completely tender and the braising liquid has reduced to a sauce that coats the back of a spoon heavily. This is where the dish lives or dies: too much liquid left in the pan produces a thin, watery result; the reduction must be pursued until the sauce is wine-dark, glossy, and concentrated.
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4
While the carbonada finishes, bring the salted water to a rolling boil. Rain in the polenta in a steady stream, whisking constantly to prevent lumps. Reduce the heat and stir with a wooden spoon for the time the polenta requires, until it pulls cleanly from the sides of the pot and the spoon stands unsupported in the mass. The polenta must remain soft enough to spread and absorb braising juices.
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5
Spoon the soft polenta directly onto each warmed plate, spreading it into a mound. Ladle the carbonada and its sauce over the polenta immediately so the juices soak in on contact. The dish is ready to serve when the sauce has visibly begun to be absorbed at the edges of the polenta.