Preheat your oven to 425 degrees F. Pat the pork tenderloin completely dry with paper towels. Dryness on the surface is what gives you a good sear. Mix the salt, pepper, garlic powder, and smoked paprika together in a small bowl, then rub the spice mixture all over the tenderloin on every side.

Heat a 10 or 12-inch oven-safe cast iron skillet over medium-high heat for about two minutes until it is very hot. Add the olive oil and swirl to coat. Lay the tenderloin in the pan and do not move it for three minutes. You want a deep brown crust. Flip it and sear the other side for three minutes. If the tenderloin is thick enough, use tongs to hold it on its side for another 30 seconds to sear the ends.

Transfer the entire skillet to the preheated oven. Roast for 14 to 18 minutes, depending on the thickness of your tenderloin. Start checking the internal temperature at 14 minutes by inserting a meat thermometer into the thickest part. You are looking for 145 degrees F. Pork tenderloin cooks fast and goes from perfect to dry quickly, so check early.

When the pork reaches 145 degrees F, remove the skillet from the oven. Transfer the tenderloin to a cutting board and tent it loosely with foil. Let it rest for at least five minutes. This rest is not optional. Cutting too soon sends all the juices straight onto the board instead of staying in the meat.

While the pork rests, make the pan sauce. Set the same skillet over medium heat on the stovetop. You should see browned bits stuck to the bottom of the pan. Add the minced shallot and garlic, stirring for about 60 seconds until softened.

Pour in the apple cider and use a wooden spoon or spatula to scrape up all those browned bits from the bottom of the skillet. That fond is flavor. Add the chicken broth, Dijon mustard, and fresh thyme. Stir to combine everything and bring the sauce to a simmer.

Let the sauce simmer and reduce for about five to six minutes until it has thickened slightly and coats a spoon. Taste it. If it feels too sweet, add the teaspoon of apple cider vinegar. Remove from heat and swirl in the cold butter until it melts into the sauce and gives it a glossy finish.

Slice the rested pork tenderloin into medallions about 3/4 inch thick. Arrange on a serving platter or individual plates over mashed cauliflower or egg noodles, and spoon the apple cider pan sauce generously over the top.