1Stem and seed the chile pods and chiles arbol. Slit them open with kitchen scissors and shake out as many seeds as possible. Rinse under cold running water. Snip into 1-inch pieces.
2Bring the water to a boil and pour it over the chile pieces in a medium heatproof bowl. Cover with plastic wrap and let stand, stirring once or twice until cool.
3With a slotted spoon, transfer the chile pieces to a blender or food processor. Process briefly, scraping down the sides. Add some of the soaking water and process again, continuing to scrape down the sides. Continue adding water until all used up and the puree is smooth.
4Transfer the puree to strainer set over a bowl. Add a couple of tablespoons of hot tap water to the blender, rinsing the blades and the inside, then pour the residue into the strainer. Force the puree through the strainer with a stiff spatula, discarding any seeds or tough bits of skin left behind.
5Position a rack in the lower third of the oven and preheat to 350 degrees.
6Lay the pork, fat side up, in a heavy, 8-quart Dutch oven. Scatter the onion and garlic slices over the roast.
7Pour the chile puree and chicken broth over the roast. Add enough water (if needed) to cover the roast half way up the side. Stir in the oregano, cumin and salt.
8Set the Dutch oven over medium heat, cover, and bring the liquid to a simmer. Then put the roast in the oven and cook it covered for about 4-5 hours, or until the meat is very tender.
9Cool the pork in its braising liquid (in the Dutch oven) set on a rack to room temperature. Transfer the roast to a cutting board, trim away any fat, and remove the bone. Using the tines of two forks, one held in each hand, in a downward pulling motion, shred the pork.
10Strain and degrease the broth. It can be used to moisten the pork for tamales, tacos, etc.