Melt the butter, then fry the carrot, celery and onion for 10 minutes, or until softened. Add the flour and cook, stirring, for 3 minutes. Add the Vegemite and tomato paste and cook for a further minute. Add the red wine and cook for 2 minutes, then gradually add the beef stock to produce a thick stew. Reduce the heat to low, return the beef to the pan with the bay leaves, thyme and sliced mushrooms. Cover and simmer for 2 hours, or until the beef is tender.