In a bowl or shallow dish, combine the flour, mustard, salt and pepper. Add steaks and turn 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 until beef is tender, 6-8 hours.
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