Heat the oil in a heavy soup pot over high heat. Add the lamb and brown lightly, stirring occasionally, for 5 minutes. Add the onions and continue to cook until the lamb is thoroughly browned and the onions are softened and colored, about 7 minutes more.
Spoon off all the fat from the pot, then add the stock, and bring to a boll. Reduce the heat to low, cover, and simmer until the lamb is tender, about 1-1/4 hours.
Refrigerate the soup, uncovered, until the fat rises to the surface, about 40 minutes. Skim the fat away.
Bring the soup to a boil over medium heat and add the turnip, zucchini, carrots, peppers, tomatoes, cumin seeds, chilis, and coriander. Cook, covered, for 20 minutes. Add the chick-peas and cook for 10 minutes more.
Remove the chili peppers from the soup and add salt and 2 tablespoons vinegar. Taste and add more salt and vinegar, if desired. Let stand, covered, for 10 minutes.