Place the onion, garlic, scallions and water in a blender and blend for 30 seconds.
Place the water mixture and beef bouillon 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 tender.
Take half of the soup and transfer to a blender and purée. Add the purée soup back to the pot, stir and cook for 20 more minutes.
Divide the soup in bowls and sprinkle cilantro on top. Serve with avocado and aji.