In a small bowl, mix together the chili powder, cumin, garlic powder, onion powder, salt, and black pepper.

Rub the spice mixture all over the chuck roast.

Place the sliced onion and minced garlic at the bottom of the slow cooker.

Place the seasoned chuck roast on top of the onions and garlic in the slow cooker.

Pour the beef broth and Worcestershire sauce over the roast.

Cover and cook on low for 6-8 hours or until the beef is tender and easily shreds with a fork.

Remove the beef from the slow cooker and let it rest for a few minutes.

Use two forks to shred the beef into bite-sized pieces.

Return the shredded beef to the slow cooker and mix with the juices.