Preheat the oven to 425 degrees

Trim all silver skin from tenderloin

Rub vegetable oil over the tenderloin and season with salt and pepper.

Heat 1 tbsp oil in a cast Iron or other oven-proof skillet over medium heat

Place tenderloin into the skillet and sear all sides, about 5 minutes each side.

Transfer pork to a large plate.

Add 1 tbsp oil to any drippings in the skillet.

Cook apples and onions in the hot oil, stirring occasionally, until onions begin to soften, about 5 minutes.

Nestle tenderloin into the skillet with apple/onion mixture.

Spread Dijon mustard evenly over browned tenderloin using a pastry brush.

Sprinkle 2 teaspoons thyme leaves over pork and apple/onion mixture

Salt & Pepper to taste

Roast in the skillet, uncovered, about 15 minutes, until an instant-read thermometer reads at least 145 degrees.

Remove skillet from the oven, place pork on a plate, cover with aluminum foil and let rest.

Add chicken stock 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.