Make the Roasted Tomatillo-Chipotle Sauce:

Drain the chiles well and put them in a blender jar. Add the garlic, cloves, oregano, salt, cumin, and ½ cup of the water and blend until smooth.

Heat the oil in a medium heavy saucepan over medium heat. Pour in the chile mixture, then rinse out the blender with the remaining ¼ cup water and add that to the pan (set the blender jar aside). Bring the sauce to a boil, stirring to incorporate the oil. Adjust the heat so the sauce is simmering.

Blend the tomatillos, with their juices, until smooth (see Notes). Pour into the sauce and stir in the piloncillo. Simmer, stirring occasionally, until the sauce is shiny and little dots of fat rise to the surface, about 30 minutes. Add water a small amount at a time if the sauce becomes too thick before turning shiny. The sauce can be prepared up to 2 days in advance and refrigerated. Bring to a simmer, adding water as necessary to restore it to the right consistency, before using.

Make the Short Ribs:

Preheat the oven to 300°F.

Trim all the fat and silver skin (the thin whitish gray membrane) from the meaty sides of the ribs. Scatter about half the celery, carrot, onion, and garlic over the bottom of a deep 13 by 9-inch casserole or roasting pan with a tight-fitting lid or other casserole in which the ribs will fit snugly. Lay the ribs over the vegetables, meat side down, overlapping them a little if necessary. (The ribs will shrink as they cook and will end up not overlapping but lying meat side down to soak up all the juices.) Tuck the pasilla chiles, if using, and bay leaves between and around the ribs. Scatter the remaining celery, onion, carrot, and garlic, the oregano, and peppercorns over the ribs. Stir the chipotle puree, salt, and water together until the salt is dissolved. Pour over the ribs and wiggle the casserole to distribute the liquid evenly. Cover the casserole with its lid or a double thickness of heavy-duty foil, crimping it tightly to the sides to make a very tight seal.

Bake until the ribs are tender when poked with a fork; the meat should pull easily away from the bone, but it shouldn’t be falling apart. This will take about 3 hours, but check at about 2½ hours. Remove the lid carefully or poke a hole in the foil to allow the steam to escape before removing the foil to check on the ribs. Replace it with new foil if further cooking is needed.

When the ribs are tender, let stand at room temperature until cool, then refrigerate for at least 6 hours, or up to 2 days.

When ready to finish the dish, wipe or scrape the solidified fat from the ribs and remove the meat from the bone. It may pull away easily or need to be cut away. Trim the cartilage that runs along the sides and back of each piece of beef. You will be left with neat little rectangles of beef. Broil or grill the ribs (see below). Whether broiling or grilling, you want to use low, steady heat to slowly heat the short ribs through while browning them. You’ll know the ribs are heated through when fat begins to sizzle on the surface. If the beef starts to brown before the ribs soften and sizzle, remove them from the broiler and lower the heat (or move the rack farther away from the heat), or wait for the coals to die down a little, and try again.

To broil the short ribs:

Position a rack about 8 inches from the broiler and preheat the broiler, to low if possible. Put the beef with what was the bone side up on the lightly oiled broiler pan. Broil until the top is browned and sizzling, about 6 minutes. Flip and repeat.

To grill the short ribs:

Heat a gas grill to low or light a charcoal fire and wait for it to die down until you can hold your hand an inch from the grill for 5 seconds. Using tongs, oil the grill with a paper towel dipped in vegetable oil. Grill the short ribs, turning occasionally, until browned, softened, and sizzling, about 12 minutes.

Meanwhile, reheat the sauce over low heat, adding small amounts of water as necessary to restore the sauce to the right consistency. Keep warm.

To serve, ladle about 1/3 cup of sauce onto each plate and center a short rib on each.