Remove any excess fat and gristle from the lamb. Heat the lard or oil in a large saucepan, add the meat and cook over high heat until evenly browned. Remove from the pan with a slotted spoon and drain on kitchen paper.
Add the onions, turnips, carrots and potatoes to the pan and cook gently, stirring a few times, for 10 minutes. Return the lamb to the pan and add the water and bouquet garni. Increase the heat and bring to the boil, then immediately reduce the heat to low and skim any scum from the surface. Cover and simmer for 1 1/2 hours, or until the meat is tender.
Using a slotted spoon, lift the meat from the saucepan and transfer it to a plate. Season the soup to taste with salt and black pepper, increase the heat and add the sliced leeks. When the soup is gently boiling again, cover the saucepan, reduce the heat and leave to simmer for a further 30 minutes.
Meanwhile, remove all the meat from the bones and cut it into small cubes. Return it to the soup for the last 10 minutes of cooking time. Skim any excess fat from the surface. Remove and discard the bouquet garni. If necessary, adjust the seasoning to taste.
Pour the soup into a large, heated tureen or six warmed serving bowls. Sprinkle the chopped parsley over the top and serve with good wholemeal bread.