Preheat the oven to 425 degrees F (220 degrees C).

Trim all silver skin from tenderloins; pat dry using paper towels. Rub 1 teaspoon vegetable oil over the surface of each tenderloin and rub with sea salt.

Heat 1 tablespoon vegetable oil in a large, oven-proof skillet over medium heat until it shimmers, about 1 minute. Place tenderloins into the skillet and cook, rotating to brown all sides, about 10 minutes. Transfer pork to a large plate.

Add remaining tablespoon oil to any drippings in the skillet. Cook apples and onions in the hot oil, stirring occasionally, until onions begin to turn translucent, about 5 minutes, adding a bit more oil if skillet gets dry.

Meanwhile, spread Dijon mustard evenly over browned tenderloins using a pastry brush. Sprinkle 2 teaspoons thyme leaves over pork. Sprinkle remaining thyme over apple mixture and add black pepper; stir gently to combine. Nestle tenderloins into the skillet with apple mixture.

Roast, uncovered, in the preheated oven until an instant-read thermometer inserted into the center of the tenderloins reads at least 145 degrees F (63 degrees C), about 15 minutes.

Remove skillet from the oven and place pork onto a plate. Cover with aluminum foil and let rest.

Meanwhile, pour chicken stock into a saucepan and cook over medium-high heat until reduced by 1/2, 8 to 10 minutes. Add to the skillet containing apple mixture.

Heat over medium-high heat until boiling, about 5 minutes; stir in butter until melted. Slice pork and serve over apple mixture.