Combine all of the ingredients for the fresh herb stuffing in a large glass bowl. Spread open the leg of lamb and pat the herb mixture on the inside of the roast. Roll lamb and tie with kitchen twine, folding in the small open end. Combine all ingredients for the Grainy Mustard Paste and pat over the surface of the rest. Set aside for 2 hours to allow flavours to develop.
Prepare the Keg for indirect grilling at 250°F, fill the drip pan with water and add the woodchips. Place the lamb roast on the grids and close the lid. Monitor that the temperature of the grill is constant at 250°F and allow the lamb to cook slowly for approximately 2 1/2 hours, until the internal temperature of the roast is 130°F for medium/rare. One half hour before removing the roast from the Keg, brush the glaze carefully over the surface of the lamb.
When internal temperature has reached 130°F, remove from Keg and cover with foil and allow the lamb to rest for 15 minutes before carving.