To make the stuffing, wash the wild rice and put it into a large saucepan in twice its volume of water. Bring the water to the boil, cover the pan and simmer until the husks have split and the rice is soft -- 50 minutes to 1 hour. Drain the rice and allow it to cool. Heat the oil in a frying pan, add the shallots and cook them over very low hear until they are soft but not brown. Add the celeriac and continue cooking until it begins to look transparent--about 3 minutes then add the spinach and cook for about 1 minute, until it wilts. Blend this mixture very briefly in a food processor to make a rough-textured puree; do not over process. Mix the puree with the wild rice, and season with the nut meg, ½ teaspoon of the salt and some black pepper.
Preheat the oven to 730'C (450"F or Mark 8) Stuff and tie the shoulder into a melon shape. Put the lamb in a roasting pan and season the outside with the remaining salt and some pepper. Roast the lamb in the oven until it is well browned - 10 to 15 minutes then reduce the oven temperature to 200 C (400 F or Mark 6) and cook for a further ½ To 1 ½ hours for medium rare to medium meat. Transfer the lamb to a carving board and allow it to rest in a warm place for 15 minutes.
While the meat is resting, make the gravy. Skim off any fat from the surface of the roasting juices and transfer the pan to the stove top. Add the stock and boil it over high heat, stirring to loosen any sediment from the bottom of the pan. Mix the cornflour with 1 tablespoon of water and add it to the pan, stirring constantly until the gravy thickens - 2 to ~ minutes. Season with black pepper. Cut off the string and carve the lamb into wedges Serve the gravy separately.
