In a large resealable plastic bag, combine the flour, mustard, salt and pepper. Add steaks and shake to coat. In a large nonstick skillet, brown steaks on both sides in butter. Transfer to a 5-qt. slow cooker. Top with onion. In a bowl, combine the tomatoes, Worcestershire sauce and brown sugar; pour over meat and onion. Cover and cook on low for 6-8 hours or until meat is tender.