Preheat grill to high.

Prick eggplant all over with a fork. Grill the eggplant turning occasionally until charred and tender 10 to 12 mins. Grill the garlic 6 to 8 mins.

Peel eggplant and garlic, transfer to food processor. Add lemon juice, tahini and salt; process until smooth. Drizzle with olive oil and sprinkle with sumac, if desired.