1.Preheat a barbecue or chargrill plate to high heat. Combine onion and half the juice in a small bowl; set aside.
2.Prick eggplants all over with a fork. Cook eggplants on a heated barbecue, turning occasionally, 30 minutes or until skin is charred and flesh is very tender. Place eggplants in a large sieve over a large bowl; drain. Cool.
3.Meanwhile, in a small bowl, combine tahini, garlic, yoghurt and remaining juice. Season to taste.
4.Remove and discard skin from eggplants. Pull flesh apart with two forks, place on a large platter; season well. Drizzle eggplant with oil. Spoon yoghurt sauce over eggplant. Top with onion mixture, sumac and mint leaves.