Place the lamb joint, whole into a large saucepan and cover with the cold water. Cover the pan and bring to a simmer. Remove any scum that forms.
Place the pearl barley into a bowl of cold water and leave to soak.
Chop the onion and add to the pan with the rosemary and thyme.
Leave to simmer for an hour, occasionally skimming off the scum.
Peel the turnips and carrots.
Chop the turnips, celery and carrots into small pieces and add to the pan. Season with salt and pepper. Simmer for half an hour.
Drain and rinse the barley and add to the pan.
Peel and chop the potatoes, add to the broth and simmer uncovered for another half an hour.
Chop the cabbage into pieces and add to the pan. Continue to cook until the lamb is falling off the bone and the liquid has a broth like consistency.
Lift the lamb out of the pot carefully. Shred the lamb from the bone and place some pieces of lamb into each bowl.
Surround the lamb with the broth.
Season with parsley, if desired.