Cut the beef joint into two pieces.
Heat a frying pan with the olive oil and sear on all sides. Place in the slow cooker along with the chopped onions.
In a mixing jug, add the remaining ingredients and stir together. Pour over the beef and onions and cool for 4 hours on high or 8 hours on low.
Remove the meat from the slow cooker, shred the meat with two forks. Mix the beef with the sauce and serve on toasted buns, salad and coleslaw.