Place all the ingredients, except the water, in a blender or food processor and blend on maximum speed for 1 minute. Before that, make sure to save a few chickpeas and some pieces of avocado for serving.
Scrape down the sides of the bowl with a silicone spatula, adjust the consistency with water, and continue mixing until smooth.
Transfer the guacahummus to a bowl, top with the rest of the chickpeas and pieces of avocado, drizzle with olive oil, and sprinkle with za'atar and coriander leaves (optional).