Position 2 oven racks in the top and middle position, and preheat to 450 degrees F. Toss the grapes and sage leaves with 2 tablespoons of the oil, 1 teaspoon salt and a few grinds of pepper on a rimmed baking sheet. Bake on the top rack until the grapes have shriveled and softened and the skins have browned, 15 to 20 minutes. Pull out some of the sage leaves to use as garnish.
Meanwhile, season the pork all over with 1/2 teaspoon salt and a few grinds of pepper. Heat the remaining tablespoon of oil in a large ovenproof skillet over high heat. Add the pork and cook, turning as needed, until well-browned on all sides, about 5 minutes. Transfer the skillet to the middle rack of the oven, and roast the pork until an instant-read thermometer inserted in the center registers 145 degrees F, 11 to 12 minutes. Set aside to rest.
While the grapes and pork roast, put the couscous in a medium bowl. Add just enough hot water to cover the couscous, cover with plastic wrap and let sit until all the liquid is absorbed and the couscous is tender, about 5 minutes. Fluff with a fork.
Slice the pork. Put the couscous on a serving platter, and top with the pork and grapes. Sprinkle with the walnuts and reserved sage leaves.
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