Heat oven to 400° F. Cook the couscous according to the package directions.
Meanwhile, heat the oil in a large ovenproof skillet over medium-high heat.
Season the pork with ½ teaspoon salt and ¼ teaspoon pepper and cook until browned, 3 to 5 minutes per side. Transfer to a plate.
Add the peaches, onion, vinegar, and ¼ teaspoon each salt and pepper to the skillet and cook, tossing, for 1 minute.
Return the pork (and any accumulated juices) to the skillet. Transfer to oven and roast until the pork is cooked through and the peaches are tender, 8 to 10 minutes.
Sprinkle the pork and peaches with the basil and serve with the couscous.