Heat oil in a large pan over medium-high heat. Season beef with salt, pepper, chili powder, cumin, paprika, oregano, and cayenne. Sear it on all sides until browned.
Place sliced onion and garlic in the bottom of a slow cooker or Dutch oven. Place beef on top of onions.
Pour in broth, vinegar and soy sauce.
Crockpot method:
Close the slow cooker lid and cook for 8 hours on the low setting or 5–6 hours on the high setting.
Oven method:
Cover and cook at 325ºF for 3–3.5 hours.
Both methods:
Once beef pulls apart easily with a fork, shred it right in the pot. I use two large forks. Stir it into the juices. Let it sit 10 minutes to soak up flavor.
Notes
Searing the beef before slow cooking adds rich, savory flavor — don’t skip it!Chuck roast works best for juicy, pull-apart shredded beef.Let the beef sit in the juices for 10 minutes after shredding to soak up flavor.You can cook this in a slow cooker or Dutch oven — both work beautifully.Leftovers freeze well — store with the juices to keep the meat tender.