Do the prep. Slice the red onion, then peel and mince the garlic. Place the beef roast into the bottom of the slow cooker. Place all of the other ingredients on top. Cover the slow cooker and cook on low for 8-10 hours or on high for 4-6 hours.
Shred and serve. Once the beef is tender and falling apart, remove it from the slow cooker and shred with two forks. Return the shredded beef to the slow cooker to absorb the juices before serving. You can also spoon off some broth to use as au jus.