How to Make It
Season the short ribs with 2 teaspoons of salt, then cover and refrigerate for at least 1 hour or up to 1 day.
Preheat the broiler and position a rack 4 to 6 inches from the heat. On a large rimmed baking sheet, toss the onions and garlic with 1 teaspoon of the oil and season with salt and pepper. Broil for 10 to 12 minutes, until the onions and garlic are charred in spots. Let cool, then discard the garlic skins. Reduce the oven temperature to 325°.
In a large enameled cast-iron casserole, heat 1 tablespoon of the oil until shimmering. Add the ancho and pasilla chiles and toast over moderate heat, turning occasionally, until they are fragrant and pliable, about 2 minutes. Add the beef stock, remove from the heat, cover and let stand until the chiles are softened, about 5 minutes. In a blender, working in 2 batches, puree the chopped tomatoes with the chocolate chips, onions, garlic and the chiles and their soaking liquid until smooth; season with salt and pepper. Wipe out the casserole.
Pat the short ribs dry with paper towels. In the casserole, heat the remaining 2 tablespoons of oil until shimmering. Add half of the ribs and cook over moderately high heat, turning once, until browned, about 8 minutes. Transfer to a plate and repeat with the remaining short ribs. Add the sauce and return the first batch of short ribs and their juices to the casserole. Bring to a simmer, cover and braise in the oven for about 2 hours, until the short ribs are very tender.
Transfer the ribs to a plate and let cool slightly, then skim the fat from the surface of the sauce. Shred the meat into bite-size pieces and discard the bones. Return the meat to the sauce and cook over moderately low heat until heated through. Serve the short ribs in warm corn tortillas and garnish with sliced avocado, radishes, pickled jalapeños, crumbled cheese, cilantro leaves and lime wedges.