Place the drained chickpeas, garlic and kosher salt in the bowl of a food processor. Process for 20 to 30 seconds. Stop, scrape down the sides of the bowl, add the tahini and process for another 15 seconds. Add the lemon juice and 1/4 cup water and process for 20 seconds, then scrape down the sides of the bowl. With the processor running, drizzle in the olive oil and process for another 20 seconds or until smooth.
To serve, transfer the hummus to a bowl and drizzle with additional olive oil and sprinkle with sumac or cumin if desired.
Mediterranean sumac is not the same plant as American poison sumac.