Place a 12-inch cast-iron skillet in the oven, then preheat the oven to 450°F.
While the oven is preheating, dry the pork tenderloin with paper towels and place it on a large plate. Sprinkle one teaspoon of Kosher salt and ½ teaspoon of freshly ground black pepper all over the tenderloin. Let stand at room temperature until ready to cook.
Wash and scrub two large Honeycrisp apples. Then, use a 16-slice apple cutter to slice the apples into ½-inch slices.2 Set the apple slices aside until ready to use and discard the core. Once the oven reaches temperature, remove the skillet and add two tablespoons of unsalted butter. Then, swirl the skillet so the butter covers the entire bottom.
Place the prepared pork tenderloin in the center of the hot skillet. Then, place the apple slices on either side of the tenderloin. And sprinkle the shallot slices over the sliced apples. Return the skillet to the oven and bake for 10 minutes.
After 10 minutes, remove the skillet from the oven and flip the tenderloin over. Stir the apple slices around a bit, and drizzle ¼ cup of apple cider over the entire pan. Return to the oven and bake for an additional 15 minutes, or until a thermometer inserted into the thickest portion reads 145°F.
Once the pork reaches 145°F, remove the pan from the oven. Transfer the pork to a large cutting board, cover with aluminum foil, and let rest for about 10 minutes before carving.
Place the skillet with the apples over medium heat and add the remaining two tablespoons of butter. Stir for about a minute, until the butter has melted. Then, add the orange zest, freshly minced thyme, ¼ teaspoon Kosher salt, and remaining ¼ cup apple cider. Bring the mixture to a simmer and stir occasionally, breaking the apples apart, for about two minutes until the mixture has reduced slightly.
Slice the pork into ½-inch slices and return it to the skillet. Garnish with sprigs of fresh thyme. Serve the pork slices with the apple sauce on top.
Yield 6 servings