PREHEAT OVEN TO 200ºC . Trim excess fat from the outside of the leg, leaving only a thin layer. Using a small knife, poke holes in the meat and stuff in slivers of garlic. Rub the surface of the lamb with olive oil. Mound onions and thyme in the middle of a roasting pan. Sprinkle the lamb with salt and pepper and place it, fat side down, on the onions. Place in the oven, reduce heat to 180ºC and roast 1 hour. Turn the lamb fat side up and return to the oven. Roast another 20-to-30 minutes for medium-rare or about 20 minutes per pound. A meat thermometer should read 60ºC for medium rare. Remove the pan from the oven and place the lamb on a cutting board. Let rest 15 minutes before carving. Scrape the onions into a food processor, add the rice, cream, nutmeg and desired salt, and pulse, scraping down the sides of the bowl, until a lumpy smooth puree is attained. When it's time to serve dinner, transfer the puree to a vegetable dish. Slice the lamb and arrange on a meat platter.