Process chickpeas, tahini, lemon juice, garlic, salt, pepper, cumin, and 2 Tbsp. water in a food processor until smooth, about 1 minute (or you can use a blender).
With the motor running, stream in 3 Tbsp. oil, then continue to process until hummus is very light and creamy, about 1 minute longer. Taste and season with salt, if needed.
Transfer hummus to a shallow bowl, top with za’atar, and drizzle with more oil.