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1
Heat a dry comal or heavy skillet over medium heat. Press the ancho, chipotle, and guajillo chiles flat against the surface for 10–15 seconds per side until they blister and release a toasted, slightly smoky scent — they must not blacken or the broth will be bitter. Transfer immediately to a bowl, cover with hot water, and soak for 20 minutes until fully pliable.
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2
Drain the soaked chiles. In a blender, combine them with the tomatoes, onion, and garlic. Blend on high to a completely smooth purée. Pass the purée through a fine-mesh sieve, pressing firmly with a spatula to discard all skins and solids. The result must be a uniform, deeply coloured sauce with no visible fragments.
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3
Melt the lard in a wide, heavy pot over medium-high heat. Pour in the strained chile purée — it will spatter — and fry, stirring constantly, for 8–10 minutes until it darkens by a shade, pulls away from the sides of the pot, and the fat begins to separate at the edges. This is where the dish lives or dies: insufficient frying at this stage leaves the base thin and raw-tasting rather than mole-like in body and depth.
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4
Pour in the water or fish stock and stir to combine. Bring to a steady simmer, then add the whole crabs in their shells. Cook at a moderate simmer for 15–20 minutes, turning the crabs once, until the shells are fully red and the broth has absorbed the oceanic depth of the shells — it should coat the back of a spoon.
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5
Add the fresh epazote leaves and the dry sherry. Simmer for a further 2 minutes. Taste and adjust salt. The epazote must be detectably green and herbal — not faded — and the sherry should be present but not dominant.