In a large stockpot, add the flank steak, onions, carrot, celery, bay leaf and peppercorns. Add water just to cover. Bring to a simmer and cook, partially covered, for about 1 1/2 hours until the beef shreds easily when pressed with a fork.
When the meat is very tender, turn off heat and let cool in liquid for 20 minutes, before removing from broth. Strain the broth into a small bowl and discard the vegetables. Set aside.
1 medium red onion
1 small green bell peppers, finely chopped
4 garlic cloves, minced
½ tsp salt
1 Tbsp 26-ounce chopped tomatoes
½ tsp ground cumin
1 tsp dried oregano
1 can chopped tomato
Make the sauce. Heat the oil over medium heat, add the minced onion, pepper and garlic. Sprinkle in the salt. Cook 8 to 10 minutes, until the vegetables begin to soften. Add the tomato paste, cumin and oregano, cooking one minute more, until the tomato paste becomes fragrant.
Stir in the meat broth and simmer 2 to 3 minutes until the broth reduces slightly. Add the tomatoes and shredded meat. Cook 20 to 25 minutes, uncovered until the liquid reduces by half. Serve with rice and black beans.