Trim the lamb of excess fat and cut into approximately 3/4-inch cubes. Set aside in a large bowl
In a small bowl, mix together the yogurt, garlic, lemon juice, coriander, oil, 1 tsp of the salt, the cayenne, and black pepper, if using. Pour over the lamb, then stir with a spoon or with your hands to get all the meat surfaces coated with marinade. Cover and refrigerate for at least 2 hours, or for as long as 12 hours
Prepare a charcoal or gas grill. Slide the lamb cubes onto 6 to 8 flat metal skewers if you have or use bamboo skewers. Grill over moderate heat, turning occasionally, until browned on the outside and succulent pale pink in the center (of cooked right through, if you prefer)
Just before serving, sprinkle about 1 tsp salt over the lamb. Slide the meat off the skewers, or serve on the skewers and allow guests to slide the meat off. Put out lime wedges so guests can squeeze on fresh juice