Heat the butter in a heavy casserole. Brown the onion to an even dark brown, but do not let it burn.
Add the lamb, pushing the onion to the sides. Add the chilies (or cayenne), salt, yogurt, coriander, cumin, ginger, and turmeric. Crush the tomatoes onto the meat and cover closely. Let the meat cook at a very low simmer until the liquid is reduced. If the liquid has not reduced after 45