In a 5- to 6-quart slow cooker, combine the tomatoes, bell peppers, onion, oregano, cumin, 1½ teaspoons salt, and ¼ teaspoon pepper. Nestle the steak among the vegetables.
Cook, covered, until the meat is tender and pulls apart easily, on high for 4 to 5 hours or on low for 7 to 8 hours.Twenty-five minutes before serving,
Using two forks, shred the beef and mix it into the cooking liquid. Serve with the rice and top with the avocado and cilantro.