Drain the garbanzo beans and give them a good rinse. For a very smooth hummus, use your fingers to pop the skin off each individual bean. If you can't be bothered, skip that step.
Tumble the beans into the bowl of a food processor. Put the lid on and pulse a few times to break up the beans.
Add the spinach, tahini, garlic cloves, and lemon juice and run the motor. While it runs, slowly stream in the olive oil and two tablespoons of water. If the hummus seems to thick, add the remaining water.
Add the salt and pepper and pulse to combine. Taste and adjust seasoning, if necessary.
Scrape the finished hummus into a container and refrigerate. It will keep about a week in the fridge.