1. Preheat oven to 375 degrees. Season pork loin with salt and pepper. Tuck sage leaves under kitchen twine along the length of the pork loin.

2. Heat olive oil in a large cast-iron skillet over medium-high heat until hot but not smoking. Add pork, and sear until golden brown on all sides, including ends, 10 to 15 minutes. Wrap pork loin in parchment paper, tucking ends under to seal completely. Place in skillet, and transfer to oven. Roast until an instant-read thermometer inserted into the center registers 150 degrees. 40 to 50 minutes. Remove pork loin from parchment paper, and transfer to a serving platter. Let rest for 10 minutes before removing twine and slicing into 1/2-inch-thick slices.

3. Place skillet over medium heat, and add cider and stock. Cook, stirring with a wooden spoon to loosen any browned bits on the bottom of the pan. Add any accumulated juices from bottom of serving platter. Continue to cook until liquid is syrupy and reduced to about 1/3 cup. Add butter, stirring until melted. Drizzle over pork, garnish with sage leaves, and serve.