This is one of those soups that for years has had a terrible reputation as the dreadful 'Brown soup' served up routinely in boarding houses before and after World War II. However, properly made this is a great British soup and well worth trying.
Melt the butter in a large pan and cook the onion for 1-2 minutes. Add the leek and carrot, cover and allow to sweat for about 5 minutes. Add the beef and stir until it is browned.
Add the flour to a little of the stock to make a paste. Add this mixture along with the rest of the stock to the pan. Bring everything to the boil. Add the bouquet garni, cover and simmer very gently for 2 hours.
Remove the bouquet garni. Liquidise the soup and serve garnished with the parsley.