Place the beef roast into a pot, either a stove top pot or slow cooker.
In a medium bowl, mix the barbecue sauce, orange preserves, mustard, brown sugar and onion. Pour this mixture over the beef
Cover the pot and cook on the stove, over low heat, for 7 to 8 hours. If you are using a slow cooker, set it to low and cook for the same amount of time, 7 or 8 hours.
To bake the bbq beef in the oven, set the oven temperature to about 300ºF. Place everything in the pot as directed above, then pop the pot into the heated oven and bake for several hours.
When it is well cooked and very tender, remove the beef from the pot. You can either cut the meat into thin slices or shred it with two large forks. Put the shredded or sliced beef back into the pot and stir it into the sauce. Cover and cook again on low for 20 to 30 minutes until it is all nice and hot.