To make the pear mustard, heat the oil in a heavy bottomed saucepan set over medium high heat. Add the pears and cook them, stirring frequently, until the juice is syrupy and lightly browned - 15 to 20 minutes. Add the stock, lemon juice, shallot or spring onion, garlic, salt, some pepper and the mustard. Reduce the heat to medium and simmer the mixture, stirring occasionally, until only about 35cl(12 floz) remain 15 - 20 minutes. Transfer the pear mustard to a food processor or a blender and puree it until smooth.
Meanwhile, preheat the oven to 180"C (350"F or Mark 4) and prepare the leg of lamb for roasting. Rub the Dijon mustard over the exposed inner surface of the leg. Fold the meat over to enclose the mustard, then tie the leg securely with string. Heat the oil in a large, shallow fireproof casserole set over high heat. When the oil is hot, add the leg of lamb and brown it evenly on all sides about 10 minutes. Sprinkle the lamb with the salt and transfer the casserole to the oven. Roast the lamb for 20 minutes.
