Spread onion slices and garlic cloves in the bottom of a slow cooker; stir in beef broth, barbeque sauce, brown sugar, cider vinegar, and pepper. Add pork roast.
Cover and cook on Low for 6 to 8 hours. Remove pork from slow cooker. Strain juices and return to the slow cooker. Mix water and cornstarch in a bowl; stir into juices until sauce is thickened.