Step 1
Preheat the oven to 425 degrees F (220 degrees C).
Step 2
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.
Step 3
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.
Step 4
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.
Step 5
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.
Step 6
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.
Step 7
Remove skillet from the oven and place pork onto a plate. Cover with aluminum foil and let rest.
Step 8
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.
Step 9
Heat over medium-high heat until boiling about 5 minutes; stir in butter until melted. Slice pork and serve over apple mixture.