Preheat the oven to 350°F/180°C/Gas 4. Soak 6-8 toothpicks in a bowl of water and set it aside.
Heat a tablespoon of olive oil in a large pan or skillet and add the onion and apples. Cook for 3-4 minutes, stirring occasionally, until the onions become slightly soft and translucent.
Remove the pan from the heat and add the walnuts, breadcrumbs, and fresh thyme. Season generously with salt and pepper to taste. Stir and set it aside.
Lay the butterflied pork tenderloin on a flat surface and cover with plastic wrap. Pound the meat with a meat mallet until around ½"/1cm thick.
Spread the apple and walnut mixture over the meat and tightly roll the tenderloin. Secure with the soaked toothpicks. Brush the tenderloin with the remaining olive oil and season generously with salt and pepper on all sides.
Place the same pan you used to cook the onion and apples on high heat. Add the rolled pork tenderloin to the hot pan. Sear for 4-5 minutes, turning until golden on all sides,
Transfer the tenderloin to the preheated oven and roast for 20-30 minutes or until fully cooked in the middle. The temperature in the thickest part of the meat should read 155°F/70°C with an instant-read thermometer.
Remove the stuffed pork tenderloin from the oven. Cover loosely with foil and allow to rest for at least 15 minutes.
Remove toothpicks, slice, and serve with your choice of sides. Enjoy!