How to Make It
In a saucepan, combine the butter, vadouvan, zest, rosemary, salt, pink peppercorns and black pepper. Cook over moderate heat until the butter is melted; let cool.
Rub the curry butter all over the lamb. Roll the boned portion of the lamb into a cylinder and tie it with butcher's twine at 3-inch intervals. Set the lamb on a rack in a roasting pan and let stand for 30 minutes.
Preheat the oven to 325°. Roast the lamb for about 2 1/2 hours, basting it every hour, until an instant-read thermometer inserted into the thickest part registers 130° for medium-rare meat. Transfer the lamb to a carving board and let rest for 15 minutes.
Pour the pan juices into a small saucepan and skim off the fat. Set the roasting pan over moderately high heat. When the drippings sizzle, add the water and bring to a boil, scraping up the browned bits. Add the deglazed drippings to the pan juices and keep hot. Carve the lamb into 1/2-inch-thick slices and serve with the curried jus.
Vadouvan is available from spicehouse.com.