Do not have the shoulder rolled by the butcher, but ask him to score the fat side for you in a diamond pattern. (If he's busy, do this at home yourself.) Combine the lemon juice, cardamom, coriander and yogurt. Spread the lamb out flat, and rub the yogurt mixture into it on both sides. Allow to stand for an hour or so, then form into a neat shape and secure meat with two long skewers. Place in a baking dish, and roast in a preheated moderate oven 180C (350F) for 30 minutes. Meanwhile, heat the butter in a small frying pan and fry the onion until soft and golden, about 4 minutes. Stir in the ginger and spread this mixture over the surface of the lamb. Continue cooking until done to your liking: about an extra hour for well-done lamb and 30 minutes for lamb still pink in the middle. Remove to a warm platter to rest for 5 minutes before carving. Blot up any fat from juices in pan, add milk and heat, scraping up the brown crusty bits from the bottom. Season with salt and pepper and serve as gravy with lamb. Serves 4.