In a skillet, melt 2 tablespoons of unsalted butter over medium heat. Add 1 finely chopped yellow onion and sauté until translucent, about 5 minutes. Stir in 2 minced garlic cloves, cooking for an additional minute, then add 1 diced Granny Smith apple, ½ cup of dried cranberries, ½ cup of chopped walnuts (if using), 1 tablespoon of fresh rosemary, 1 tablespoon of fresh thyme, and ½ teaspoon of ground cinnamon. Cook for another 2-3 minutes, then mix in 1 cup of fresh breadcrumbs and enough chicken broth to moisten, about ¼ cup. Set aside to cool.

Using a sharp knife, carefully butterfly a 1.5-pound boneless pork tenderloin by slicing it horizontally, ensuring not to cut all the way through. Open it like a book and pat it flat with your hand or a meat mallet, then season generously with salt and freshly ground black pepper on both sides.

Spread the cooled filling evenly over the butterflied pork tenderloin, leaving about an inch at the edges. Starting from one end, carefully roll the tenderloin tightly to encapsulate the filling, creating a cylindrical shape. Secure the roll with kitchen twine or toothpicks.

Heat 2 tablespoons of olive oil in a large oven-safe skillet over medium-high heat. Once hot, place the rolled tenderloin seam-side down in the skillet. Sear for about 3-4 minutes on each side until golden brown.

Preheat your oven to 375°F (190°C). After the pork is seared, carefully transfer the skillet to the preheated oven and roast for about 30-35 minutes, or until the internal temperature reaches 145°F (63°C).

Once the pork tenderloin is cooked, remove it from the oven and let it rest on a cutting board for at least 10 minutes. After resting, slice it into rounds to reveal the colorful filling.