Transfer the grill grate from a charcoal grill and build a hot fire with lump charcoal until red-hot. Rub the cabbage lightly with olive oil and season generously with salt, ensuring that all sides are well coated. Put on your fireproof gloves. Make a well in the center of the coals using a rake or shovel, and carefully add the cabbage. Use the rake or shovel to bury the cabbage completely by covering it with the surrounding hot coals. You may grill something on top at the same time if desired. Close the lid to keep the heat in and to prevent the charcoal from burning out too quickly. You want to maintain the temperature at about 400°F by adjusting the vents.
The cabbage should be completely charred and black on all sides and tender in the center in about 1 hour; check by piercing with a cake tester or paring knife. If it’s not done, continue roasting in the coals for 5 more minutes. If you are preparing the cabbage ahead, wrap tightly in aluminum foil and keep it warm in a low oven.
Prepare the sauce: In a small mixing bowl, combine the yogurt, chives, lemon zest, lemon juice, and sumac until smooth.
Cut the cabbage through the core into quarters. Cut the core out of each piece. Arrange the cabbage wedges on a serving platter and season lightly with salt. Rip off the cabbage leaves with your fingers and dip into the sauce.