With the food processor running, drop in the garlic and pulse one or twice until minced. Add the chickpeas with one-half of the reserved liquid, the tahini, sumac, salt, pepper and lemon juice. Process until smooth. With the processor running, slowly add the oil in a steady stream.† Hummus should be about the consistency of yogurt. If too thick add some of the remaining chickpea liquid, adjust seasonings to taste.
Serve on a shallow dish or plate, garnished with small Greek olives.