Heat oven to 425 degrees. Tuck 6 sage leaves under kitchen twine and season pork with salt and pepper.
Heat 1 tablespoon oil in a large ovenproof skillet over medium-high heat. Sear pork on all sides until golden, about 8 minutes, then remove from heat and set aside. Add remaining oil to pan with grapes, shallot, and remaining sage. Season with salt and pepper. Nest pork in center of pan.
Transfer pan to oven and roast until pork reaches 145 degrees on an instant-read thermometer, about 15 minutes. Remove pork and let stand 10 minutes before slicing.
Stir vinegar into grape mixture. Spoon grape mixture over pork and serve with couscous.
Give It a Rest
Before slicing, let the meat sit for 10 minutes to better distribute the juices.