In a large Dutch oven, warm the oil over medium-high heat.
Add the bell peppers, onion and garlic, season with salt and pepper and cook until lightly caramelized, for 10 to 15 minutes; transfer the vegetables to a bowl.
Pat the flank steaks with paper towels to remove any excess moisture and season generously with salt and pepper.
Working one at a time, sear the steaks on both sides; transfer to a plate.
Add the broth, and with a wooden spoon, scrape up any brown bits.
Add the tomato sauce, tomato paste, cumin and vinegar and stir to combine; bring to a simmer.
Return the steaks and vegetables to the Dutch oven, cover with a lid and cook until very tender, about 4 hours.
Use a fork to shred the flank steaks into shreds.
Season with salt and pepper, to taste; serve.
Notes
The stringiness of the pulled flank steak is why ropa vieja translates to “old rags.” Braised flank steak has a little more bite and chew than you may be used to. If you’re looking for a more tender cut, swap out the flank for chuck.