Preheat the oven to 400 degrees F (200 degrees C). Line a baking sheet with parchment paper.

Remove silver skin and any surface fat from tenderloin; dry well with paper towels. Combine Cajun seasoning, celery salt, garlic granules, onion powder, and sugar in a small bowl; mix well. Sprinkle seasoning mixture over pork tenderloin.

Heat oil into a large skillet over high heat until shimmering. Add pork tenderloin and sear for 3 to 4 minutes per side. Transfer to the prepared baking sheet and place into the oven.

Roast in the preheated oven until pork is slightly pink in the center, about 15 minutes. An instant-read thermometer inserted into the center should read at least 145 degrees F (63 degrees C). Check temperature of the pork tenderloin after 10 minutes as baking time depends on how long it has been seared.

Meanwhile, pour apple cider and apple cider vinegar into the skillet and bring to a low boil. Scrape up any browned bits, lower heat, and simmer for 15 minutes. Add maple syrup and simmer for 5 more minutes.

Move pork tenderloin to a clean cutting board, cover, and let rest for 10 minutes.

Slice pork tenderloin into thin slices, place on a serving dish, and drizzle with apple cider au jus.