Preheat the oven to 300ºF (150ºC).
In a large, heavy, ovenproof pot, such as a Dutch oven, stir together the onion, garlic, bell peppers, diced tomatoes with their juice, mustard, raisins, and olives (hang onto that liquid from the olies for later).
Sprinkle the chuck roast all over with the salt, pepper, and cumin. Nestle the pieces into the mixture in the pot. Cover the pot tightly with a lid or aluminum foil.
Roast the beef until it’s incredibly tender and shreds easily when you poke at it with tongs or a couple of forks, 3 to 3 1/2 hours.
Add the olive liquid to the pot and use those tongs or forks to shred the beef directly in the pot (discard any large pieces of fat as you work).
Stir the beef and juices together to combine. Taste and, if desired, add more salt. Serve warm with cilantro, if desired. (It’s honestly better the longer it sits. You can refrigerate it for up to a week and warm it up in a saucepan over low heat, splashing in a little water or stock if it needs some moisture. You can also freeze it for up to 3 months.)