Aguachile negro

Place dried morita, ancho, and árbol chiles directly over an open gas flame or in a dry cast-iron pan. Char them thoroughly on all sides until the skins are genuinely black and brittle — not toasted, not browned. Char the serrano chiles alongside. The smell must shift from toasted to acrid and smoky. This is the step on which the dish lives or dies: without true char, the sauce stays brown and the signature black colour and smoky depth are lost.

Transfer all charred chiles to a blender. Add the lime juice and black soy sauce or Worcestershire. Blend until completely smooth. Add the black sesame oil and pulse once to combine. The sauce must be deeply black and liquid enough to coat the prawns; add a small amount of cold water if the texture is too thick to pour.

Butterfly the raw prawns with a sharp knife, opening them flat without cutting through. Arrange in a single layer in a cold, wide dish. Pour the black aguachile over the prawns so each one is fully submerged or evenly coated.

Cure for 3–5 minutes maximum. The prawns are ready when the flesh has turned from translucent grey to opaque-pink at the edges while remaining slightly yielding at the centre — they must not feel firm or rubbery to light pressure. Remove from the cure the moment this point is reached.

Arrange the ice-cold cucumber and red onion slices across tostadas. Lay the cured prawns over the vegetables, spoon the remaining black cure around and over, and serve immediately. The dish must reach the table within 10 minutes of curing; beyond this point the texture degrades.