In a large skillet, add the vegetable oil and heat over medium-high. Add the flank steak and brown each side for about 3-4 minutes each.
Place the steak into a slow cooker, add the tomato sauce, beef broth, tomato paste, bell pepper, onion, garlic, cilantro, cumin, bay leaf, vinegar and olive oil. Mix and stir well all ingredients. Cover the slow cooker and cook on high for to 4 hours, or, you can set it up on low, and cook for about 10 hours.
2Before serving, let the meat cool down enough to handle, and shred it with two forks into thin slices. Put meat back into the sauce and spoon meat and sauce over white rice and a great side of fried sweet plantains.