1. Cut a slit in the side of each garlic clove. Place them in a microwaveable bowl, cover with plastic wrap, poke holes in the top and microwave for 30 seconds on high. Cool until handleable, then slip off the papery husks.
2. One by one, drop the garlic cloves into a running food processor, letting each get thoroughly chopped before adding the next. Add the chile powder, cumin, black pepper, vinegar, 1 teaspoons salt and 1/2 cup water and pulse to blend.
3. Place the meat in a large crock pot. Scrape the marinade onto the meat, then spread it sloppily over the top and sides. Pour enough water into the slow-cooker to cover the lower 1/2 inch of roast. Cover and slow cook on high for 6 hours or until meat is tender. Carefully remove meat with a slotted spoon onto a large plate, pulling away the bones.
4.Degrease the cooking broth and boil it down in a saucepan to concentrate the flavors. Ladle it over the meat and garnish with the onions and cilantro.
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