Preheat the oven to 425 degrees F. Place the lamb in a roasting pan. Brush with mustard. Sprinkle with pepper, salt and 1 Tbsp of the rosemary. Roast 20 minutes. Reduce heat to 325 degrees F and cook lamb to desired degree of doneness, checking its progress with a meat thermometer. 75–90 minutes should yield medium rare in the middle. Insert a meat thermometer into the thickest part of the leg to determine degree of “doneness.” When inserted into the centre of the thickest part of the leg it should read 135–140 degrees F for medium rare, 160 degrees F for medium, and 170 degrees F for well done. Rest the lamb 10 minutes before slicing thinly. To make jus, remove excess fat from roasting pan and pour in stock and add remaining rosemary. Simmer 5 minutes, then season to taste and serve.