1. Position racks in upper and lower thirds of oven; preheat to 475 degrees F.
2. Toss apples, fennel and onion with 1 tablespoon oil in a large bowl. Spread out on a rimmed baking sheet. Roast on the lower oven rack, stirring twice, until tender and golden (30-35 minutes).
3. While apples are cooking, sprinkle pork with salt and pepper. Heat the remaining oil in a large, ovenproof skillet over medium-high heat. Sear the pork on one side, about 2 minutes.
4. Turn the pork over and transfer the pan to the top oven rack while apples are still roasting. Roast until just barely pink in the center and an instant-read thermometer registers 155 degrees F (12-14 minutes).
5. Transfer the pork to a cutting board and let rest for 5 minutes.
6. While pork is resting, stir vinegar into the skillet, scraping up browned bits, then add to the apple mixture.
7. Slice the pork thinly and serve with the apple mixture, garnished with fennel fronds.