I see this question out there, but I don't think you'll really find that this particular recipe needs thickened. That's one of the benefits of using the bean liquid instead of water or broth. In fact, when this soup cools, it thickens up quite a bit, and then thins back out a little to a more soup-y consistency when you reheat it. If you really do want it even thicker, though, you could puree or mash some of the beans to add body without needing to add any other ingredients.