In a food processor, pulverize the chickpeas and garlic. Stop and scrape down the bowl occasionally.
Add the avocado and process until smooth. Add the lemon juice, tahini, and salt and process until smooth. If needed to lighten the hummus, add cold water, chickpea canning or cooking liquid, 1 tablespoon at a time until smooth and spreadable.
Dollop the hummus into a bowl and create a well in the middle with the back of a clean spoon. Drizzle with olive oil, dust with sumac, and serve immediately with pita chips and vegetables.