1Position racks in upper and lower thirds of oven; preheat to 475°F.
2Toss apples, sliced 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 to 35 minutes.
3About 10 minutes after the apple mixture goes into the oven, sprinkle pork with salt and pepper. Heat the remaining 2 teaspoons oil in a large ovenproof 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 oven rack. Roast until just barely pink in the center and an instant-read thermometer registers 145°F, for 12 to 14 minutes.
4Transfer the pork to a cutting board and let rest for 5 minutes. Immediately stir vinegar into the pan (be careful, the handle will be hot), scraping up any browned bits, then add to the apple mixture. Thinly slice the pork; serve with the apple mixture and sprinkle with fennel fronds.