Heat the butter in a large saucepan, add the beef and onion and fry for 5 minutes, stirring, until the beef is browned and the onion just beginning to colour.
Stir in the diced vegetables, pearl barley, stock and mustard, if using. Season with salt and pepper and bring to the boil. Cover and simmer for 1¾ hours, stirring occasionally until the meat and vegetables are very tender. Taste and adjust the seasoning if needed. Ladle the soup into bowls and sprinkle with a little chopped parsley. Serve with warm potato bannocks or farls.