1. Preheat broiler and lightly oil a shallow baking pan. Arrange lemon slices in pan, placing 2 inches apart. Rinse chicken and pat dry. Evenly sprinkle all sides with herbes de Provence; season with salt and pepper. Place each chicken thigh, skin side down, on a lemon slice and broil 4 inches from heat, turning once, until cooked through, about 17 minutes.
2. Meanwhile, bring 1-3/4 cups water and 1 tablespoon oil to a boil in a medium saucepan. Add couscous in a slow stream, stirring until combined. Immediately cover pan and remove from heat. Let stand 5 minutes. Using a fork, stir in zest, parsley, celery, olives, and remaining 1 tablespoon oil; salt and pepper to taste.
3. Arrange chicken with lemon slices on one side of serving plates and couscous on the other. Makes 6 servings.