Cuba's national dish, named 'old clothes' for the long, fibrous strands flank steak shreds into after a slow, barely-bubbling braise - a hard boil would toughen the lean cut around the edges, so the sauce is kept at the gentlest simmer throughout.
1.Simmer the flank steak in water with a pinch of salt at medium-low heat until very tender, about 2 hours, then remove and let cool slightly, reserving the cooking liquid.
2.Using two forks, shred the beef along its natural grain into long, fibrous strands - this is what gives ropa vieja ('old clothes') its characteristic rope-like texture and name.
3.Heat the olive oil in a large pot and sauté the onion and bell peppers until softened, then add the garlic and tomato paste, letting the paste cook for a minute or two until it darkens slightly and loses its raw edge.
4.Pour in the wine and scrape up any browned bits, then stir in the crushed tomatoes, beef broth, cumin, and bay leaves.
5.Add the shredded beef to the sauce and simmer gently, keeping the heat low enough that the sauce barely bubbles, since a hard boil toughens the lean, shredded meat around the edges.
6.Simmer for 20-30 minutes until the flavors meld, then stir in the olives, capers, and vinegar.