Place roast in a 5-qt. slow cooker. Top with onion. Combine the ketchup, root beer, vinegar, Worcestershire sauce, hot sauce, salt, pepper and mustard; pour over top. Cover and cook on low until meat is tender, 6-8 hours.
Meanwhile, scrub and pierce potatoes. Bake at 400° until tender, 50-55 minutes.
Remove pork; shred meat with 2 forks and set aside. Skim fat from cooking juices; transfer juices to a large saucepan. Bring liquid to a boil. Combine cornstarch and water until smooth; gradually stir into the pan. Return to a boil; cook and stir until thickened, 2 minutes. Return shredded meat to cooking juices; heat through.
With a sharp knife, cut an “X” in each potato; fluff with a fork. Top each with butter, pork mixture and sour cream. Sprinkle with cheese and, if desired, green onions.