Preheat the oven to 500°. In a roasting pan that's large enough to hold the lamb, spread out the vegetables, herbs and peppercorns. Season the lamb generously with salt. Set the lamb on top of the vegetables and roast for about 25 minutes, until the lamb is lightly browned.
Add the stock to the pan and cover the pan with foil. Reduce the oven temperature to 300° and braise the lamb for 2 hours. Uncover the lamb and cook for 1 hour longer, until deeply browned on top and the meat is very tender. Let the lamb rest in the juices for 15 minutes, then transfer it to a carving board. Strain the cooking juices, discarding the solids, and spoon off the fat. Slice the lamb 1/4 inch thick and serve with some of the cooking juices.