Step 1

Position racks in upper and lower thirds of oven; preheat to 475 degrees F.

Step 2

Toss 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.

Step 3

About 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 degrees F, 12 to 14 minutes.

Step 4

Transfer 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.