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1
Pat the lamb pieces thoroughly dry. Melt the lard in a wide, heavy-based pan over a high heat. Brown the lamb in batches — do not crowd the pan. Each piece should develop a deep mahogany crust on all sides before it is moved. The fat will spit; the crust, not the steam, is what builds the sauce's backbone. Remove each browned piece and set aside.
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2
Reduce the heat to medium. Add the garlic and crumbled peperoncino to the residual fat in the pan. Cook briefly until the garlic is pale gold and fragrant — it should not darken further.
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3
Return all the lamb to the pan. Pour in the white wine and wine vinegar in equal measure — 125 ml of each. The acid should rise sharply on the nose as it hits the hot pan. Stir, scraping up any residue from the base. The liquid should come roughly a third of the way up the meat.
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4
Braise over a low, steady heat, covered, turning the pieces occasionally, until the lamb is tender through to the bone and yields without resistance when pressed. This is the body of the cook; the sauce at this stage will still be loose.
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5
Remove the lid. Raise the heat to medium. Add the rosemary sprigs now — not before. Braise uncovered for 20 minutes, turning the lamb as the sauce reduces. It is ready when it coats the back of a spoon and clings visibly to the lamb pieces with a glossy, concentrated sheen.
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6
Off the heat, stir in the tablespoon of red wine vinegar. The acid finish should be detectable and bright on the palate — not a background note. Taste and correct the salt. Discard the rosemary sprigs before serving.