If making with lamb shoulder, rub all over with sea salt and white pepper, place in a roasting tray, cover with tin foil and roast for 3 hours, or until falling off the bone. Pull all the meat off the bone, saving the bone for later.
Wash, trim and roughly chop the leek, peel and finely slice the onions, then trim and roughly chop the carrots and celery.
Place the leek, celery, onions and carrots into a large pan on a low heat with 1 tablespoon of oil, and cook gently for 20 minutes, or until soft and sweet, stirring regularly.
Pour in the stock, and add the reserved lamb bone (if you have it). Bring to the boil, then simmer gently for 20 minutes, while you peel and roughly chop the swede and potato.
Add pearl barley to the pan, followed by the swede and potato, then simmer gently for a further 50 minutes, or until tender.
Remove the pan from the heat and whisk the broth quite hard to break down some of the potato and bind the soup together, then stir in the roast lamb or cooked lamb, to heat through. Taste and season to perfection with salt and pepper.
Pick the parsley leaves, then sprinkle over the broth with the reserved celery leaves (if using). Serve with hunks of crusty bread for dunking, and if you want to be really authentic, why not add a little dram of whisky to each bowl?