3 lb ( 1.5kg) pork leg or loin, boned and cut into chunks
1 onion, halved and thinly sliced
2 cloves garlic, minced (finely chopped)
4 tablespoons (2 fl oz/60 ml) vegetable oil
2 pasilla chiles
3 arbol chiles
1/2 cup (4 fl oz/125 ml) water
1 1/2 cups (12 fl oz/375 ml) flat light beer
1/2 teaspoon salt, or to taste
Cook the pork in salted water to cover for 50 minutes, or until cooked through and tender. Strain and discard the stock.
In a large flameproof casserole, saute the onion and garlic in the oil until translucent.
Lightly roast chiles over an open flame. Remove the stems and seeds and blend them in a blender with the water until smooth. Add the mixture to the casserole. Rinse the blender with beer the beer and add to the casserole
along with the chunks of pork and the salt.
Cook over low heat for about 10 minutes, or until the sauce is reduced and thickened and the meat starts to fall apart.
Serve at room temperature, in the casserole or on a serving platter with hot corn tortillas to make tacos.