To make the stock, put the lamb shanks, 3 onions, 3 turnips, 2 carrots, peppercorns and water in a large pot. Bring to a boil, reduce the heat and simmer, covered, for 3 hours. Skim the surface as required.
Remove the shanks and any meat that has fallen off the bones and cool slightly. Remove the meat from the bones and finely chop, then cover and refrigerate. Strain the stock, discarding the vegetables. Cool the stock and refrigerate overnight, or until the fat has set on top and can be spooned off.
Cover the barley with water and soak for 1 hour.
Put the stock in a large pan and gently reheat. Add the drained barley, extra carrot, onion, leek, celery and turnip. Bring to a boil, reduce the heat and simmer for 30 minutes, or until the barley and vegetables are just cooked. Return the meat to the pan and simmer for 5 minutes. Season well and serve with parsley.
