Combine the chickpeas, tahini, water, lemon juice, olive oil (if using), cumin, garlic, and salt in the bowl of a food processor.
Purée until completely smooth, stopping occasionally to scrape down the sides of the bowl with a spatula. This may take 3-5 minutes for the best texture.
Add the fresh parsley and pulse for just 5-10 seconds to incorporate it without turning the hummus green.
Taste and adjust the seasoning as needed, adding more salt or lemon juice if desired.
Serve immediately or store in an airtight container in the refrigerator.