Cut roast in half; sprinkle with pepper. Place onion and roast in a 5-qt. slow cooker. In a small bowl, combine the beer, ketchup and brown sugar; pour over top. Cover and cook on low for 8-10 hours or until meat is tender.
Remove meat to a serving platter; keep warm. Skim fat from cooking juices; transfer to a small saucepan. Bring liquid to a boil.
Combine flour and water until smooth; gradually stir into the pan. Bring to a boil; cook and stir for 2 minutes or until thickened. Serve with roast.