Step 1 Stir together the yogurt and vinegar. Add the garlic, squeezed through a press or smashed, then minced with a chef's knife. Stir in the black pepper, ginger, serrano chile, cumin and salt.
Step 2 Place the lamb in a resealable plastic bag, then pour in the yogurt mixture. Press out all the air, seal tightly and massage to distribute the marinade evenly. Let stand at room temperature 1 to 2 hours.
Step 3Start the coals in a chimney and when they are lightly coated with gray ash, about 20 minutes, empty them into the grill, arranging them in a gentle slope against one side. Replace the grill rack and brush it with oil.
Step 4Place the lamb skin-side down directly over the flame and sear for 2 to 4 minutes. Turn and sear the other side. Move the lamb away from the direct heat and cook until it is brown and crusty on the outside and has reached an interior temperature of 130 degrees, about 25 minutes.
Step 5Transfer the lamb to a carving board and let rest 5 to 10 minutes to allow the juices to redistribute before carving in thick slices. Serve immediately.