Cut the flank steak into large pieces and place them in a large bowl. In a small bowl, combine the adobo and the Worcestershire sauce to make a paste. Rub the paste into the meat, add the beer, and let marinate for 15 minutes in the refrigerator.
Transfer the meat, as well as the marinade to a large pot and bring to a boil over medium heat. Reduce the heat to low, cover, and simmer until the meat is tender, about 3 hours.
Remove the meat from the pot and reserve the cooking liquid. Using 2 forks, shred the meat and set it aside.
Heat the oil in a large deep skillet over medium-high heat. Add the onions, peppers, scallions, and tomatoes and cook until they are soft, 10 minutes. Add the tomato paste and meat to the pan along with 2 cups of the reserved cooking liquid. Cover and simmer until the juices cook down, about 15 minutes.
Combine all ingredients.
Recipe courtesy of Ingrid Hoffmann, 2008