01 - Layer the sliced onions and minced garlic evenly across the bottom of the slow cooker insert.
02 - Whisk together the beef broth, tomato paste, and Worcestershire sauce in a small bowl until the tomato paste is fully dissolved.
03 - Combine the chili powder, cumin, smoked paprika, oregano, salt, black pepper, and cayenne pepper in a small dish. Pat the chuck roast dry with paper towels, then rub the spice mixture thoroughly over all surfaces of the meat.
04 - Place the seasoned chuck roast directly on top of the onion and garlic bed in the slow cooker.
05 - Pour the broth mixture evenly over the roast and aromatics.
06 - Cover with the lid and cook on low heat for 8 hours. The beef is done when it shreds effortlessly with a fork.
07 - Transfer the cooked roast to a clean cutting board. Using two forks, shred the meat by pulling in opposite directions. Return the shredded beef to the slow cooker and toss with the accumulated juices.
08 - Serve hot in tortillas, over rice, or on sandwich rolls. Store leftovers in an airtight container in the refrigerator for up to 4 days.