Preheat the oven to 300˚F.
Set an ovenproof pot or Dutch oven over medium-high heat, add a drizzle of oil and brown the roast on all sides.
Add the onions to the pan and cook for a few minutes, until starting to brown.
Add the stock, barbecue sauce, brown sugar, balsamic vinegar and mustard, cover and cook for 3-5 hours, or until the meat is very tender.
Using two forks, pull the meat apart in the sauce and serve on soft buns.