16-pound semi-boneless leg of lambtrimmed of excess fat
Sprigs of fresh cilantro leaves
Hot couscouscooked according to package directions, to serve
In a food processor or blender combine garlic, shallots, parsley, cumin, paprika, coriander, allspice, salt, pepper, ginger, cayenne, olive oil, lemon juice and honey. Pulse to combine, then puree until it forms a paste. Use a sharp knife to cut about 15 deep slits into the lamb on all sides, then rub the mixture all over the lamb, working in into the slits. Place the lamb into a roasting pan, cover it loosely with plastic wrap, and refrigerate for at least 6 hours, preferably overnight.
Let the lamb sit out at room temperature for an hour. Preheat the oven to 225°F. Place the lamb in a rack in a roasting pan. Roast the lamb in the oven for about 2½ to 2¾ hours, until it reaches an internal temperature of 125°F on an internal thermometer. Turn the heat up to 500°F. and let the meat cook for another 15 to 20 minutes (the temperature in the oven will climb, but may not quite reach 500°F within 15 minutes, which is fine). Check the internal temperature: for rare you want to take it out at 125°F, for medium rare you want about 135°F, and medium is 140 to 145°F. The temperature of the lamb will climb for a few minutes once the meat is taken out of the oven. Remove it from the oven, transfer the lamb to a cutting board, tent it with foil, and let the meat sit for 30 minutes.
While the meat is resting, in a small bowl combine the harissa with the crème fraiche and season with salt. Thinly slice the meat across the grain and pile it onto a platter. Serve with the cilantro sprigs and the creamy harissa sauce.