Fold lamb into a compact, relatively uniform shape. (If your lamb is in the oven-safe netted bag used by most supermarkets, remove the bag; most of the outside crust will fall off if you remove it after roasting.) Tie in about 5 spots using kitchen string; securing it. Make 4 to 6 gashes in the meat , about 1/4 inch deep. Place the roast in a shallow pan.
To prepare marinade: Combine yogurt, 2/3 cup ginger, garlic, cinnamon sticks, peppercorns and salt in a blender. Puree, scraping down the sides as needed, to make a slightly gritty paste, speckled with spices .
Pour the marinade over the lamb and massage it into the meat, making sure you get it into the gashes. Cover and refrigerate for at least 4 hours or up to 24 hours.
Preheat oven to 425ºF. Line a roasting pan with foil, place a roasting rack in the pan and spray with cooking spray. While the oven heats, let the lamb stand at room temperature.
Transfer the lamb to the prepared rack. (Reserve the marinade left behind.) Add 1 cup hot water to the roasting pan. Roast the lamb for 15 minutes. Baste with the reserved marinade.
Reduce the oven temperature to 325°. Continue roasting until an instant-read thermometer inserted into the center of the roast registers 135ºF (for medium-rare), 1 3/4 to 2 1/4 hours more.
To prepare couscous: About 20 minutes before the lamb is done, heat 2 tablespoons oil in a large nonstick skillet or saucepan over medium heat. Add bell pepper and 1/4 cup ginger and cook, stirring, for 2 minutes. Stir in couscous, salt and pepper; cook, stirring, until the couscous is lightly toasted , 3 to 5 minutes. Add kale and broth (or water); the pan will be full. Cover, reduce the heat to low, and simmer , stirring once or twice, until the liquid is absorbed and the couscous is tender , 12 to 15 minutes. Fluff with a fork and spread the mixture on a large serving platter.
Let the meat rest on a clean cutting board for 10 minutes. Cut off the string, cut the lamb into 1/2-inch-thick slices and arrange on the bed of couscous.
Tip: Israeli couscous is small, pearl-shaped pasta made from semolina flour (not to be confused with tiny-grained, quick-cooking couscous). Look for it near other Middle Eastern dry goods in well- stocked supermarkets or specialty foods stores. If you can’t find it, orzo can be used as a substitute.
To Make Ahead: Marinate the lamb (Steps 1-3) up to 24 hours ahead. | Equipment: Kitchen string