Make the lamb: Preheat the oven to 350°. In a small bowl, mix together the curry powder, cinnamon and coriander. Set the lamb on a cutting board and if the roast is tied, remove and discard the string. Rub the spices all over both sides of the lamb, then rub with 1 tablespoon of the salt. Roll the lamb into a tidy roast and use butcher's twine to retie the lamb in 1½-inch intervals. Place the lamb in a baking dish and roast until an instant-read thermometer inserted into the thickest part of the lamb reads 140°, about 1 hour and 10 minutes. Remove from the oven, loosely tent with aluminum foil and set aside to rest for 15 minutes.
While the lamb rests, make the couscous: Bring the water to a boil in a medium saucepan. Add the couscous and the salt. Stir, cover the saucepan and turn off the heat. Let the couscous absorb the water until it is fluffy and tender, about 5 minutes. Remove the lid and use a fork to stir in the preserved lemon.
In a medium bowl, mix together the crème fraîche, ras el hanout (if using) and remaining ½ teaspoon of salt. Carve the lamb crosswise into thin slices and serve with the couscous and a dollop of the spiced crème fraîche.