Position a rack in the upper third of the oven and preheat to 425 degrees . On a rimmed baking sheet, arrange the chicken pieces skin side up. Stuff 1 prosciutto slice under the skin of each chicken piece. Sprinkle the lemon juice on top. Quarter 1 tablespoon butter and place one quarter on each chicken piece; season with salt and pepper. Roast until the juices run clear when a thigh is pierced, about 35 minutes.
Meanwhile, prepare the couscous according to package directions. In a large skillet, bring 1/2 inch water to a boil. Add half of the spinach and wilt over medium-high heat for 1 minute. Add the remaining spinach and wilt for 1 minute more. Drain and squeeze dry. Wipe out the skillet.
In the same skillet, melt the remaining 1 tablespoon butter over medium heat. Add the onion and cook, stirring, until softened, about 5 minutes. Whisk in the flour and then the cream and bring to a boil. Stir in the lemon peel and spinach and remove from the heat. Season with salt and pepper. Just before serving, discard the lemon peel.
Fluff and mound the couscous onto plates. Top with the chicken and serve the spinach on the side.