1Put the tomato sauce, water, bay leaves, vinegar, garlic, cumin, salt and black pepper into the crock pot. Stir to combine.
2Peel the onion and cut it into quarters. Thinly slice the onion quarters, and add them to the crock pot.
3Place the beef over the onions, and spoon some of the liquid over the meat. (If you are using skirt steak or flank steak, there will be several thin pieces, and they will overlap in the pot, but you could use sirloin or chuck).
4Rinse bell peppers and discard the seeds and membranes.
Cut the peppers into quarters, thinly slice them, and add the pieces to the crock pot.
5Pour the diced tomatoes with their juice evenly on top.
6Cover the pot and cook on low until the meat is so tender that it practically falls apart, about 8 hours if using skirt or flank steak or 10 hours if using sirloin or chuck.
7About 20 minutes before serving, cook the rice.
8Before serving, removed the beef and vegetables from the crock pot to a large serving bowl.
Use two forks to pull the beef into shreds. There should be some cooking juices in the bowl, but if not, add about ½ cup juices.
9Stir to mix the beef and vegetables.
Season with additional salt and pepper if desired.
Serve all but two cups of the beef vegetable mixture over hot rice.
10Refrigerate leftovers, skim off fat and store in the refrigerator for up to to 3 days or a month in the freezer.