Wash and scrub potatoes and dry thoroughly with a paper towel. Rub each potato with olive oil and sprinkle with 1/2 teaspoon salt and 1/2 teaspoon pepper. Wrap each potato in a piece of parchment paper and set aside.
Place the sliced red onions in a 5-6 quart slow cooker that has been coated lightly with cooking spray.
Prepare the barbecue rub by combining the brown sugar, tomato paste, Worcestershire sauce, chili powder, 1 teaspoon salt, and 3/4 teaspoon pepper. Spread half the rub on top of the roast.
Place the roast on top of the onions in the slow cooker with the coated side down. Pour the rest of the barbecue rub on top of the roast and spread until covered completely.
Place the wrapped potatoes on top of roast. Cover and cook on HIGH 5 hours or until tender. (Or may cook on LOW for 8 hours.)
Unwrap potatoes; cut a slit in each potato and fluff with a fork.
Shred the roast with 2 forks, and stir to combine with onion mixture.
Top each potato with the BBQ beef mixture, cheese, sour cream, and green onions.