In the bowl of the food processor, add the beans, tahini, harissa, garlic, and lemon juice. Process for a few seconds to get the mixture started. While the processor runs, begin drizzling in some of the additional seasoned oil from the jar of harissa, starting with 2 tablespoons, adding more as necessary to create a smooth paste. Total processing time will be about 3 to 5 minutes for best texture.
The flavor is best with garbanzo beans cooked at home, but you can use a 15-ounce (425 g) can of garbanzo beans in a pinch.