In a pan over medium heat melt 2 tbsp butter, then add in onion and celery. Fry until soft and just about starting to take on colour, then add in apple, garlic, sage and thyme. Continue frying until the apple begins to soften, then stir in 2 tsp dijon mustard.
Add in breadcrumbs, pecans and a good pinch of salt and pepper. Stir to combine, then stir in 1-2 tbsp stock or water, just until it begins to stick together ever-so-slightly. A little tacky if you will. Place pan to one side.
Grab your pork tenderloin and lay it on a chopping board. Slice lengthways through the centre around halfway down, then open up the tenderloin so it's even thickness. Flip it over then lay a sheet of cling film over and pound until around 1/2" thick. You want it fairly thin, but not so thin it starts breaking apart. Remove cling film.
Lay over strips of bacon from one end right to the other, overlapping very slightly as needed. Place fresh cling film back on top, the press down and secure around the edges of the chopping board. Carefully flip it over so the clingfilm is on the work surface and the pork is on top (see video for guidance). Season with a good pinch of salt and pepper.
Add the stuffing in a row towards the bottom of the tenderloin (side closest to you). Tightly roll just the tenderloin, then bring back to the start of the bacon and tightly wrap the rolled tenderloin in bacon. Secure with toothpicks if the seam looks loose.
Place seam-side-down in a baking tray and roast at 220C/425F for around 35mins, or until the centre of the meat hits 63C/145F. If the meat cooks before the bacon crisps up, whack on the grill and broil until it crisps. Timings will vary slightly depending on the thickness of the tenderloin so I recommend using a thermometer.
Rest for 3-5mins, then slice up and enjoy!