Position racks in the upper and lower thirds of the oven; preheat to 475
Toss apples, sliced fennel, and onion with 1 Tbsp oil in a large bowl. Spread out on a rimmed baking sheet. Roast on the lower rack, stirring twice, until tender and golden about 30-35 minutes.
About 10 minutes after the apple mixture goes into the oven, sprinkle pork with salt and pepper. Heat remaining 2 tsp oil in large oven proof skillet over medium high heat. Sear the pork on one side, about 2 minutes. Turn the pork over and transfer the pan to the top rack. Roast until just barely pink in the center and a meat thermometer registers 145, 12-14 minutes.
Transfer the pork to a cutting board and let rest for 5 minutes. Immediately stir the vinegar in the pan scrapping up any browned bits, then add the apple mixture. Thinly slice the pork and serve.