We developed this thick, garlicky yogurt sauce sprinkled with tart sumac to pair with eggplants that had been roasted directly on a bed of hot coals. They get so smoky and silky, that this cold, creamy sauce paired perfectly—especially when served with grilled naan. That said, you can use it on just about anything. Feel free to experiment with other seasonings besides sumac, like lemon zest, za'atar, or crushed red pepper flakes.