Place a rack in the lower third of the oven and heat the oven to 425°F.
Heat the oil or butter in an 8-quart Dutch oven or other heavy ovenproof pot over medium heat. Add the onions, sprinkle over the salt, and cook, stirring occasionally, until soft but not colored, about 8 minutes. Add the turmeric and stir to coat the onions.
Add the chickpeas, along with several grinds of black pepper. Nestle the lamb in the chickpeas and add enough hot water to almost submerge the lamb. Bring the liquid to a boil, cover the pot, place it in the oven, and cook for 45 minutes.
Remove the pot from the oven. Some of the liquid will have evaporated and some of the lamb shanks should have begun to brown. Turn the lamb, cover the pot, and return to the oven for another 45 minutes.
Remove the pot from the oven. There should still be enough liquid to partially submerge the chickpeas; if there isn’t, stir in 1 cup hot water. Arrange the tomato slices over the top of the stew, return the uncovered pot to the oven, and cook for 30 more minutes.
Check the stew: The tomatoes should be soft and may have browned a bit, the lamb should be tender, and the chickpeas should be cooked through. (Return the stew to the oven and cook for a bit longer if necessary.) Serve immediately, or set it aside, covered, for up to 2 hours (bring the liquid back to a boil before serving). Or cool in the pot, transfer to an airtight container, and refrigerate or freeze.