Position a rack in the upper third of the oven and preheat to 450 degrees F. Cook the couscous as the label directs, omitting any oil or butter. Fluff with a fork and toss with 2 tablespoons olive oil, and salt and pepper to taste. Cover and keep warm. Meanwhile, put the chicken and onion on a rimmed baking sheet and toss with the remaining 2 tablespoons olive oil, the herbes de Provence, 1 1/2 teaspoons salt and 1/2 teaspoon pepper. Arrange the chicken skin-side up and roast until the chicken begins to brown, about 15 minutes. Remove the baking sheet from the oven and scatter the grapes on top. Continue roasting until the chicken is golden brown and the grapes are soft, 15 to 20 more minutes. Divide the chicken, onion and grapes among plates and drizzle with the pan drippings. Serve with the couscous.