Preheat oven to 425⁰ F.
Use paper towel to pat pork tenderloin dry, then use a sharp knife to remove the silverskin.
Rub pork with 1 tablespoon oil and the salt.
Heat 1 tablespoon olive oil in cast iron or another oven-safe skillet over medium heat for 2-3 minutes, until oil shimmers. Sear pork on all sides until golden brown,about 12 minutes total.
Meanwhile, chop 1 tablespoon fresh thyme, core, peel and slice 2 apples and slice 2 onions.
Once pork is browned, remove and set aside. Add in apples and onions, cooking for 5 minutes, then add in thyme.
Add pork back to pan on top of the apple and onion mixture. Bake for 10-15 minutes, or until the internal temperature of the pork reaches 145⁰ F.
Remove pork to cutting board, and let rest for 10 minutes. Meanwhile place skillet back on stove-top over medium heat. Add chicken stock, scraping any bits of deliciousness from the bottom of the pan. Bring mixture to a simmer, and cook until reduced by half. Add in butter, stir until incorporated into sauce.
Slice, pour sauce over pork, and serve immediately.