The key to smooth, luscious hummus is to make every ingredient the best it can be. Peeled chickpeas are essential to the texture of your hummus, which you can get here. To skin your own canned or cooked chickpeas, saute the chickpeas over medium heat with a touch of baking soda, then put them in a bowl with lots of water. Agitate and rub the chickpeas with your fingers and pour off the water with skins; repeat several times. This recipe is adapted from my cookbook, Rose Water & Orange Blossoms: Fresh and Classic Recipes from My Lebanese Kitchen.
In the bowl of a food processor, puree the chickpeas and garlic until a thick paste forms. Give this a couple of minutes (longer than you'd think), to get the chickpeas very well pureed. With the machine running, add the tahini, salt, and lemon juice. Taste and see if you'd like the hummus thinner or lighter. If so, Slowly add cold cooking liquid or water, a tablespoon at a time, until the hummus is very smooth and light but still thick (again, process a little longer than usual). Taste and adjust with salt and lemon juice as needed.
Spoon the hummus onto salad plate or bowl, spreading the hummus with the back of a spoon to form a well in the center. Drizzle with olive oil and sprinkle with sumac, and serve immediately.