Cook the beans:
Drain the beans and combine all of the ingredients except the salt in a large, heavybottomed pot. Fill the pot with water 2 inches above the top of the beans and bring to a gentle simmer over low heat. Cook until the beans are soft and tender, about 2 to 3 hours. Resist the temptation to turn up the heat: Low and slow is best for beans. Season the cooked beans to taste with a generous pinch of salt. Cool the beans in their liquid. Using a blender or food processor, purée the beans until smooth. If you want an even smoother purée, pass the mixture through a fine-mesh strainer. Reheat the beans gently over low heat, adding water as needed.
Prepare the steak:
Lay the steak lengthwise in front of you on a cutting board. Going with the grain, cut the steak in half. Take each half of steak and slice it in half horizontally starting at the cut side and moving your knife slowly and carefully through the middle of the steak towards the edge, leaving about ¾ inch of the meat connected.
Preheat the oven to 450°F. Coat the plantains lightly with olive oil and place them cut side down on a baking sheet. Roast until they are nicely browned, 15 to 20 minutes. Let them cool completely on the pan. Keep the oven on.
Heat a medium pot of salted water over high heat. Blanch the sliced bell peppers briefly (about 45 seconds) then shock them immediately in an ice water bath to cool completely. Dry thoroughly.
Assemble the steak:
Lay each piece of steak on a cutting board and open it up like a book. Season the inside generously with salt and pepper. Lay 2 halved plantains lengthwise on the steak and top with the green and red peppers. Roll the steaks up tightly lengthwise like a cigar; tie the rolled steaks every 2 inches with butcher’s twine. Trim the ends of the rolls. Season the outside of the steak rolls generously with salt and pepper.
Heat oil in a large skillet over high heat until shimmering. Sear the steak rolls on all sides until well browned. Transfer the skillet to the oven and cook to medium rare, about 10 to 15 minutes longer. Let rest for 10 minutes.
Slice the steak between the pieces of twine into 2-inch-thick pieces. Serve the steak with a spoonful of the bean purée and a sprinkling of chopped cilantro.