Place ¼ cup of water, onion, scallion, ground cumin and garlic in the blender and blend until well combined.
Remove excess fat from the oxtails. Rinse the oxtail pieces well with cold water. Place in a large pot and cover with the water. Add the onion mixture, salt and pepper. Bring to a boil and then cover the pot and lower the heat to medium and let it cook for about 2 hours or until the oxtails are tender. Check the water level during cooking and add a more if necessary.
Add the carrots, peas and green beans. Season with salt and pepper to your taste. Reduce the heat to low and simmer for about 15 minutes. Add the rice and continue simmering for 15 minutes more or until the rice is tender. Add the fresh cilantro and serve.