In a pot, boil the beef in salted water until very tender (approx. 2-3 hours). Let it cool, then shred it into thin strands.
In a skillet, sauté the chopped onion, bell pepper, and garlic. Add the chopped tomatoes, salt, and pepper. Simmer for 10 minutes. Stir in the shredded beef, add a little of the reserved cooking liquid, and let it simmer for 15 minutes to combine the flavors.
If using dried beans, soak them overnight. In a large pot, boil the beans in fresh water with onion and garlic. Once the beans are soft, drain them and return to the pot. Add salt and cumin. Create a thicker sauce by mashing some of the beans or adding a little water.
Cook the rice according to the package instructions. Peel the plantains, slice them thinly, and fry in hot oil until golden-brown and crispy.
On each plate, arrange a mound of rice, a serving of shredded beef, a portion of black beans, and a few fried plantain slices. Serve immediately and enjoy the combination of flavors.