Chilpachole de jaiba

Step 1: If using live 2 ⅝ lb live blue crabs or brown crab, chill them in the freezer for 20 minutes so they are dormant, then plunge them into 6 ¼ cups fish or shellfish stock at a rolling boil and cook until the shells are bright orange-red.

Step 2: Lift out the crabs and keep the stock. When cool enough, pull off the top shells, remove the grey gills and halve the bodies; crack the claws. Pick some white meat for the soup and keep everything cold. If using cooked crab, just crack the claws.

Step 3: Roast 3 tomatoes, 1 white onion and 4 cloves garlic in a dry heavy pan over a medium-high heat until blackened in spots and soft. Peel the garlic. Toast 2 dried guajillo chillies for 20 seconds a side and soak it in a cup of the hot stock.

Step 4: Blend the roasted vegetables, guajillo with its liquid and 2 chipotle chillies in adobo until very smooth.

Step 5: Heat 3 tbsp vegetable oil in a large pot over a medium-high heat. Pour in the purée and fry, stirring, until it darkens and thickens, about 5 minutes.

Step 6: Add the stock and bring to a simmer. Whisk in 2 tbsp masa harina and simmer, stirring, until the soup is lightly thickened and coats a spoon. Add 3 sprigs epazote and 1 tsp salt.

Step 7: Add the crab pieces and meat and heat through for 3–4 minutes, just until piping hot. Don't boil the crab for long: that is the usual reason the meat turns stringy and dry.

Step 8: Remove the epazote, ladle into deep bowls and serve with 2 lime wedges and 4 corn tostadas.