In a large saucepan melt lard over moderate heat. Add pork, salt and pepper and simmer until tender. Cook about an hour, being careful not to crisp the meat. Remove pork and set aside until cool. Shred the meat by hand or with tines of a fork. Remove and discard any remaining fat. Transfer to a medium saucepan, add the arbol salsa and cook over moderate heat, stirring frequently. Serve or assemble within a toasted corn tortilla to make taco. Carnitas can stay refrigerated for up to five days before assembling within a taco.
Preheat broiler. Place tomatoes and tomatillos on a baking sheet. Broil, turning occasionally until charred all over, 10 to 12 minutes. Transfer to a saucepan. Add the remaining ingredients.
Bring to a boil and cook until onions are soft, 12 to 15 minutes. Transfer to a food processor or blender. Puree and then strain. Serve at room temperature or slightly chilled. Arbol Salsa can be stored in the refrigerator 3 to 5 days, or frozen for weeks.
c.1997, M.S. Milliken & S. Feniger, all rights reserved