In a large pot, add oxtails, season with salt and pepper, add the onions, scallions, garlic and marinate at least 4 hours or overnight in the refrigerator, stirring occasionally.
Add enough water to just cover the oxtails and bring the mixture to a boil. Lower to a simmer, cover and cook for about 2 hours, add more water if necessary.
dd the broth, tomatoes and beef broth. Bring to a boil then simmer for about 1 hour and 30 minutes, stirring occasionally.
Add the carrots, potatoes, cumin and achiote. Cook for 30 minutes more or until everything is tender. Sprinkle on fresh cilantro and serve with white rice.