Slice the onions and place on the bottom of your slow cooker.
Cut beef roast into 3 large chunks, and transfer to the slow cooker with the onions.
Mix together all the remaining ingredients in a medium bowl until well combine and pour over beef and onions. Toss to coat evenly.
Place lid on slow cooker and cook on low for 8-10 hours or until meat shreds easily.
Remove from slow cooker and shred with 2 forks, then return and toss with thes sauce and onions.