With a sharp knife, cut deep slits in the lamb and insert the strips of ginger. Rub with thyme and season with salt and pepper.

Heat the butter with the olive oil in a flameproof casserole large enough to take the double joint of lamb. When the foaming subsides, brown the lamb, fat side down first, for 4 minutes on each side. Remove the meat from the casserole and keep hot.

Add the quartered onions and finely chopped garlic and cook over low heat for 7—10 minutes, or until the onion is soft.

Add the beef stock, saffron with the soaking water, cinnamon, paprika, cayenne pepper, ground ginger and lemons. Season to taste with salt and freshly ground black pepper. Bring to the boil. Return the lamb, fat side down, to the casserole. Cover and simmer for 45 minutes or until the lamb is tender, basting frequently.

Turn the joint over and cook for a further 10 minutes, to ensure that the neck of lamb is cooked evenly. Transfer the lamb to a serving platter, arranging it like an arch, and keep hot.

Remove the lemons from the casserole with a slotted spoon and discard. Place casserole over high heat and boil until the liquid is reduced to half the original quantity (about 425 ml (15 fl oz)). Pour the sauce into a sauce-boat. Garnish the lamb with watercress, tucking it under the arch at each end. Serve immediately. To carve, cut down between the cutlets.