Place the ribs in a large pot with the cumin, salt, pepper and the water.
Bring to a boil, reduce heat and simmer for about one hour. Place the garlic, onions and scallions in a blender with ¼ cup of water and blend for about 1 minute. Add this mixture to the pot and cook for 40 minutes more.
Add half of the cilantro, potatoes and carrots if using. Season with salt and pepper and cook for 25 to 30 minutes more or until the potatoes are tender.
Sprinkle with chopped fresh cilantro and serve warm.