Remove stems, seeds and veins from the chiles.

Carefully toast the pods, in a dry iron frying pan until soft and pliable but not until burnt.
(If the chiles burn they will become bitter.)

Simmer the chiles in the chicken broth along with the garlic, oregano, cumin, and salt for 20 minutes.

Run chiles and broth through the blender one-half cup at a time.
(Be careful hot liquids expand in a blender and can spill out the top and burn you.)

Work sauce through a wire sieve with the back of a spoon into a mixing bowl, until only the chile solids are left in the sieve.

Discard chile solids.

Trim any excess fat from the pork roast and cut into large chunks.

Brown the pork in the oil and place in a crock-pot with the sauce for 5 or 6 hours on low.

Serve with hot corn tortillas and ice-cold beer.