In a mortar and pestle or food processor, pound the garlic, salt, cumin, and black pepper to a paste. Mix in the vinegar, and then blend in the olive oil. Transfer the mixture to a large shallow earthenware dish, and place the lamb in the marinade, turning to evenly coat all the chunks. Marinate at room temperature for 2 to 3 hours.
Remove the lamb from the marinade with a slotted spoon, reserving 1/2 cup of the marinade. In a large heavy casserole, heat the 3 tablespoons olive oil over high heat. Brown the lamb, turning with tongs to sear all sides evenly. This will take about 20 minutes (if the lamb does not fit all in one layer, brown in 2 batches, otherwise the meat will steam rather than sear). Transfer the lamb to a plate with a slotted spoon and set aside. Reduce the heat to medium-high and add the jalapenos, onions, and salt. Saute, stirring frequently, for about 10 minutes, or until the onions are golden. Add the beer and lower the heat so that the liquid is simmering. Cook for about 15 minutes, or until all of the beer has evaporated. Return the browned lamb, along with the accumulated juices from the plate, to the pan. Add the stock and stir to mix. Bring up to a boil, then reduce the heat and simmer the dish, covered, for about I hour and 20 minutes, or until the meat is tender.
Add the potatoes, cover the pan again and cook for about 20 minutes more, or until the potatoes are tender (you may need to add a little more stock or water to keep them from scorching). Add the reserved 1/2 cup of the marinade and the cilantro, stir to mix, and cook for 1 minute more. Serve from the casserole.
c.1997, M.S. Milliken & S. Feniger, all rights reserved