Spray the liner of a slow cooker with cooking spray. Place the roast and the onion in the bottom of the slow cooker. In a small bowl, whisk together remaining ingredients, except for buns. Pour over beef.
Cover and cook on low for 8-10 hours, or until beef is tender. Remove meat from the pot. Skim any fat from cooking liquid. Shred beef with two forks and return meat to slow cooker. Cover and cook for an additional 15 minutes or until heated through.
Place meat on sandwich buns and serve.
Notes
Cooking just for two?This cooked barbecue freezes really well, so stash away any leftovers in airtight containers and save for later meals.