Place the onion, garlic, scallions and one cup of broth in a blender and blend for about 30 seconds.
Place the broth mixture in a medium pot and bring to a boil. Reduce the heat to medium and add the rest of the ingredients, except the cilantro.
Cover the pot and reduce the heat to medium low and cook for 30 minutes or until the plantains are very tender.
Transfer the soup to a blender and puruée. Add the puruée soup back to the pot, stir and cook for 10 more minutes. Add more broth if necessary
Divide the soup in bowls and sprinkle with fresh cilantro on top. Serve with avocado and aji.