Heat oven to 220C
Trim each tenderloin of any silver skin, just use a small sharp knife and slide the blade under and outward to remove it.
Pat pork dry with paper towels and then rub with 1 tablespoon of the oil and 1 1/2 teaspoons of salt.
Heat a tablespoon of oil in a heavy-bottomed oven-safe fry pan over medium heat. When the oil is hot and shimmers, add the pork and cook, turning occasionally, until evenly browned all over; about 12 minutes. Transfer to a large plate or cutting board. (The pork will not be cooked through).
Keep the pan used to sear the pork on the stove over medium heat. Check the pan, if it looks dry, add 2 to 3 teaspoons of additional oil. (If there is fat left in the pan from cooking the pork, there is no need to add additional oil).
Add the apples and onions then cook, stirring 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 the 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 65 and 70 degrees C. Transfer the pork to a large plate and cover with aluminium 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.
To serve, slice pork into 1-inch slices then serve on a bed of t