Start the beans (best done first): Soak black beans overnight. Drain, add to a pot, cover with fresh water, add 1/2 onion and 1 garlic clove. Simmer 1 1/2 to 2 hours until soft. Remove onion and garlic, salt to taste. In a small pan, sauté a spoonful of diced onion in oil, stir into beans, simmer 10 more minutes.
Cook the beef: Place beef in a large pot, cover with water, salt generously. Bring to a boil, then reduce to a gentle simmer, cover, and cook 2 to 2 1/2 hours until very tender. Remove beef and reserve about 1 cup of cooking liquid. Shred beef into thin strands.
Build the carne mechada base: In the same pot, heat oil. Sauté diced onion and bell pepper until soft. Add garlic briefly, then tomatoes, cumin, pepper, and bay leaf if using. Return shredded beef and splash in reserved broth until juicy. Simmer uncovered 15 to 20 minutes, stirring, until moist but not soupy.
Cook the rice: Heat oil in a pot, add rice and stir briefly. Add water and salt, bring to a boil, cover, reduce to low, cook 15 minutes, then rest off heat 10 minutes.
Fry the plantains: Peel and slice plantains diagonally. Fry in shallow oil until golden and caramelized, drain.
Assemble: Plate rice, beans, shredded beef, and plantains together. Eat them in the same bite whenever possible. That is the whole point.