1. Preheat oven to 400 degree F. Rinse and pat lamb dry. Season with salt and pepper; let stand at room temperature 1 hour.
2. Heat 2 tablespoons oil in a heavy roasting pan placed over 2 burners over moderately high heat. Brown lamb on both sides, about 7 minutes. Turn lamb fat side up and cut 1-inch slits in top about 1-1/2 inches apart. Push garlic slices into slits. Combine honey, juice, rosemary, and 2 tablespoons oil in a bowl; reserve for basting. Roast lamb in middle of oven, basting halfway through with honey mixture (adding water, as needed, to bottom of pan if dry), until an instant-read thermometer inserted by 2 inches into thickest part of lamb (not touching bone) registers 125 degree F for medium, 1 to 1-1/4 hours. Transfer lamb in pan to rack; let stand 15 minutes before carving into 1/4-inch-thick slices.
3. Meanwhile, bring 2-1/2 cups water to boil with chopped garlic and stir in couscous. Immediately cover and remove pan from heat. Let couscous stand, covered, 5 minutes. Add remaining 1 tablespoon oil, the pistachios, apricots, currants, scallion, and zest; salt and pepper to taste. Stir with a fork to combine, breaking up any clumps of couscous.
4. Serve lamb with couscous and spoon any pan juices over meat. Makes 8 to 10 servings.