1. Preheat oven to 400°F.
2. In a large saucepan over a medium heat, Melt butter and cook the apple and shallot until softened, about 4-5 minutes. Add port wine and cook until evaporated.
3. Press the apple with a potato masher to break up. Turn off the heat.
4. Add the rest of the ingredients to the pan. Cook for about another minute or two. Set aside to cool.
5. On a chopping board, slice the tenderloin lengthwise about ¾ of the way through the tenderloin.
6. Spread the filling evenly across the tenderloin. Fold the tenderloin over and tie it up with butchers’ twine.
7. Wrap tenderloin in plastic wrap and let it set in the fridge for an hour or so or overnight.
8. Place pork on a roasting rack in a pan, season with salt and pepper. Roast for about 50 – 60 minutes or until internal temperature reaches 150°F. Let rest for 10 minutes before slicing.