Preheat the oven to 350°F.

Soak the 6-8 toothpicks in a bowl of water. This is to prevent the toothpicks from burning in the oven.

Heat a tablespoon of olive oil in a large heavy skillet on medium heat. Add in the onion and apple slices. Cook, stirring occasionally until the onions become slightly soft and translucent. Do this for around 3-4 minutes.

Remove from heat and add in the walnuts, breadcrumbs, and fresh thyme. Season generously with salt and pepper to taste. Set aside.

Lay the pork tenderloin on a flat surface and cover it with plastic wrap. Pound the meat with a meat mallet until around 1/2 inch thick.

Spread the diced apples and walnut mixture over the meat and tightly roll the tenderloin like a jelly roll. Secure the walnut and apple stuffing inside the tenderloin with toothpicks or kitchen string. Brush the tenderloin with the remaining olive oil and season generously with salt and pepper on all sides.

Place the same skillet you used to cook the onion and apples on medium-high heat.

Add the pork tenderloin to the hot skillet and sear until golden on all sides, around 5-6 minutes.

Transfer the tenderloin to the preheated oven and roast until fully cooked through in the middle for 20-30 minutes. The internal temperature in the thickest part of the meat should read 155°F with an instant-read thermometer.

Remove the stuffed loin from the oven. As the pork rests for 15 minutes, cover it loosely with foil. Remove the toothpicks, slice, and serve with a few remaining walnuts on top and a fresh veggie of your choice. Enjoy!