Use a sharp knife to slice lengthways through each tenderloin (taking care not to cut the whole way through) and then open it out like a book.
Season the meat and scatter the herbs over the cut sides, then close them again. Lay half the pancetta slices on a sheet of clingfilm and place one piece of pork on top. Bring the clingfilm up and over the pork, so it is wrapped with the pancetta. Repeat with the other tenderloin. Remove the clingfilm, place on a lightly greased baking sheet and cook in a preheated oven, 200°C (400°F), Gas Mark 6, for 20–25 minutes until the meat is cooked through.
Meanwhile, heat the oil in a frying pan. Add the onion and cook for 5 minutes, then stir in the apple and cook over a medium heat for 3 minutes more until softened and browned. Add the Calvados or brandy to the pan, allow to bubble away, then pour over the apple juice. Cook until reduced by half. Stir in the crème fraîche and season to taste.
Cut the pork into thick slices and spoon over the sauce. Serve with mashed potatoes and watercress.