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1
Place the pork in a pot, cover with cold salted water, add the first portion of onion, and bring to a simmer. Cook until the meat is about half-done — tender at the surface but still firm at the bone. Remove the pork; strain and reserve all the broth.
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2
Blend tomatoes, garlic, and the second portion of onion to a rough purée.
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3
Heat the lard in a wide, heavy pot over medium-high heat. Brown the pork pieces lightly, then pour in the tomato purée. Fry, stirring frequently, until the sauce darkens in colour and the fat begins to separate at the edges, about 10–12 minutes.
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4
Blend the serrano chiles with enough of the reserved broth to make a pourable liquid. Add the guajes and blend again briefly — the mixture should remain coarse and granular.
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5
Pour the guaje mixture over the pork and tomato base. Add additional broth to reach a saucy but not watery consistency. Tuck in the cilantro sprigs.
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6
Reduce heat to a gentle simmer — around 90°C / 195°F — and cook uncovered for approximately 45 minutes, adding broth as needed to maintain consistency. Adjust salt. Serve with warm tortillas or white rice.