In a large pot, bring 5 cups salted water, the broth and thyme to a boil. Add the chicken, lower the heat and simmer, uncovered, for 7 minutes. Remove the pot from the heat, cover and let stand for 15 minutes. Transfer the chicken to a cutting board and let rest for 5 minutes, then cut into 1/2-inch-thick slices. Reserve the poaching liquid.
In a medium bowl, combine the grapes, onion, basil, olive oil, lemon juice and half the lemon peel. Season with salt and set aside.
Return the poaching liquid to a boil. Pour 1 1/2 cups boiling liquid over the couscous, cover and let stand for 5 minutes. Add the remaining lemon peel and fluff with a fork. Place the chicken over the couscous and top with the grape relish.