Heat oven to 425℉. Trim each tenderloin of any silver skin (this can be tough when cooked). To do this, use a small sharp knife and slide the blade under and outward to remove it.
Pat loins dry with paper towels and then rub with one tablespoon of the oil and 1 1/2 teaspoons of salt. Heat a tablespoon of oil in a heavy-bottomed oven-safe skillet (we use cast iron) over medium heat. When the oil is hot and shimmers, add the tenderloins and cook, occasionally turning, until evenly browned all over; about 12 minutes. Transfer to a large plate or cutting board.
Keep the pan on medium heat and add the apples and onions. Stir occasionally until lightly browned around edges, about 5 minutes. Stir in 1 teaspoon of thyme.
Use a pastry brush (or use your fingers) to rub the seared pork all over with the mustard, 2 teaspoons of thyme, and fresh cracked black pepper. Place the seared pork tenderloins on top of the apples and onions, and then slide into the oven.
Roast 10 to 15 minutes or until an internal thermometer inserted into the thickest part registers between 145 and 150 degrees F. Transfer the pork to a large plate and cover with aluminum foil. Let rest about 10 minutes.
While the pork rests, place the pan with apples and onions back onto the stove and turn heat to medium. Add chicken stock and use a wooden spoon to scrape the pan, lifting any brown bits from the bottom. Bring to a simmer and cook until reduced by half. Add butter and stir until melted.
Enjoy and share your finished meal on @usetillsocial