Place the onion, garlic and scallions in a blender with ¼ cup of water and blend.
Place the plantains, lentils, onion mixture and broth in a pot and bring to a boil. Then reduce the heat to medium low, cover and cook until the lentils become tender, but not mushy, about 35 minutes. Season with salt and pepper.
Add the carrots and cumin, cook for about 15 minutes or until the carrots are tender, thin with water if necessary. Add fresh cilantro and serve.