Preheat oven to 450 degrees F. In small bowl stir 1 tbsp butter, 1 tbsp chopped sage, 1/2 tsp salt, 1/4 tsp pepper. With hands rub this mixture all over pork.
In 15x10 jelly-roll pan melt the remaining butter in oven. Remove pan from oven. Add potatoes, onion, 1/2 tsp salt, and remaining chopped sage. Toss to coat. Push potatoes and onion to edge of pan and set small roasting rack in center. Place pork on the rack. Roast 20 minutes.
Add apples to pan around the pork, and roast 30 to 40 minutes longer, depending on the thickness of pork, or until meat thermometer inserted into center of pork reaches 145 degrees F.
Transfer pork to a warming platter; let stand 15 minutes to set juices. Transfer potatoes, onion, and apples to platter around the pork. Pour pan drippings into a small bowl; skim off fat from drippings. Serve with pork.