Season the pork on both sides with sea salt and freshly ground black pepper and set aside.
Cook the apple in butter, without stirring for a few minutes until it starts to caramelise. Toss and cook for a few minutes more until caramelised all over. Remove from the pan and set aside.
Add the chopped bacon or pancetta to the same pan and fry for 3-4 minutes. Add the onion and garlic and gently fry with the pancetta until soft.
Add caramelized apples, thyme and bread crumbs, mix and combine, fry for 1-2 minutes. Set aside to cool slightly.
Make a 2.5 cm deep incision down the length of the tenderloin; do not cut all the way through. Open the meat like a book so the tenderloin lies flat. Pound with the flat side of a meat mallet until about 1 cm thick.
Spread the apple mixture evenly along the middle and starting with a long side, tightly roll up the tenderloin. Secure with metal or wooden toothpicks.
Heat the olive oil in a frying pan and brown the pork on all sides for 4-5 minutes. Then place the pork in the oven and cook for about 50 minutes on 160°C, basting with its juices from time to time. When the pork is cooked, remove the meat from the pan and rest in a warm place for 15-20 minutes, covered with foil.
Meanwhile, for the balsamic reduction, heat the balsamic vinegar in a small pan and simmer to reduce to about half. It will have a syrupy consistency. Add smoked paprika, jam and meat broth, stir to combine. Cook some more to obtain the desired consistency.
Serve the pork loin in slightly overlapping slices, with a little of the sauce poured over along and with the rest of the sauce served in a warm jug.