Prepare the stuffing: In a skillet, sauté diced onion and celery in melted butter over medium heat until softened, about 5-7 minutes. Add diced apple, minced garlic, fresh sage, and thyme; cook until the apple is tender, around 3-5 minutes. Stir in Dijon mustard, breadcrumbs, and chopped pecans, then season with salt and pepper. Moisten the mixture slightly with chicken stock.

Butterfly the pork: Slice the pork tenderloin lengthwise down the center without cutting all the way through. Open it up and pound it gently to achieve an even, uniform thickness.

Add bacon: Lay strips of bacon over the butterflied pork, ensuring they overlap slightly, covering the entire surface.

Fill and roll: Spoon the prepared stuffing at one end of the pork. Roll tightly, tucking in the sides as you go, and wrap it securely in the bacon. Use toothpicks to secure the ends if needed.

Roast: Preheat your oven to 425°F (220°C). Place the stuffed pork, seam-side down, in a baking tray and roast for approximately 35 minutes, or until a meat thermometer reads 145°F (63°C).

Finish: Remove from the oven and let the rolled tenderloin rest for 3-5 minutes. Slice into medallions to serve.