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1
Bring a large pot of unsalted water to a full boil. Add the pork rind strips and blanch for 5 minutes. Drain and discard the water. The rinds should be visibly defatted and firm; any grey foam that has risen confirms excess lard has been removed.
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2
Warm the lard or olive oil in a heavy pot over a medium flame. Add the onion, celery, carrot, and garlic. Soften slowly, stirring occasionally, until the vegetables are translucent and yielding — 8 to 10 minutes. They should not colour.
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3
Add the drained soaked beans, the blanched cotiche strips, and the piece of pancetta or pork lard to the pot. Stir to combine with the softened vegetables.
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4
Add the chopped tomatoes and pour over the water or light stock until everything is generously covered. Season lightly with salt — the pork and pancetta will release further salt throughout cooking. Bring to the barest simmer.
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5
This is where the dish lives or dies: maintain the most gentle simmer possible — the surface should tremble, not boil — for 2 to 2.5 hours. A rolling boil will fracture the beans and deny the cotiche the sustained, low heat they need to surrender their collagen. At the 2-hour mark, press a bean against the side of the pot; it must yield completely with no chalky resistance, and a strip of cotiche, when lifted and pressed between two fingers, must feel soft, slippery, and gelatinous rather than rubbery or fibrous.
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6
Add the sprig of rosemary in the last 30 minutes of cooking. Remove and discard it before serving; its fragrance should be present but not dominant in the finished soup.
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7
Remove the pancetta piece. Adjust the salt with care. The soup should be thick enough to coat the back of a spoon and leave a clean trail when a finger is drawn across it — neither a thin broth nor a stiff purée. Ladle into bowls, drizzle with extra-virgin olive oil, and serve immediately with rough country bread.