Shred some of the lamb fat and render it down in a heavy casserole. Peel onions and potatoes, scrape carrots. Cut the meat into 8 pieces; only the excess fat is cut away. Bones need not be removed. Cut the carrots and onions in quarters. Toss meat in fat until color changes, and repeat with onions and carrots. Add stock and season carefully. Put whole potatoes on top. Simmer gently until the meat is cooked, 2 hours approx. Pour off the cooking liquid. Degrease, and reheat in another saucepan. Check seasoning. Then swirl in butter, chives, parsley, and pour back over stew.