Preheat the oven to 180°C (350°F/Gas 4). Halve the bread horizontally. If there is a lot of soft bread under the crusts, pluck out a little. Put most of the herbed salt on a board with 2 tablespoons of olive oil and rub all over the meat. Heat a non-stick frying pan to very hot. Sear the meat on all sides until it’s a good colour, turning it carefully so you don’t burn the herbs. Remove from the pan and nestle it in between the 2 halves of bread. Scatter with the remaining salt, then close it up like a sandwich and squash it down a bit. Trim away the ends of the bread so the loaf is the same length as the pork. Drape the pancetta over the top and sides. Tie with string in 4 or 5 places so the loaf will hold its shape.
Line a baking tray with baking paper. Drizzle another tablespoon of the olive oil onto the bottom. Sit the bread fillet on top and drizzle with the remaining tablespoon of olive oil. Roast for about 30 minutes or until it is golden and crusty on top, and when poked with a skewer the meat releases juices that are clear, not pink. Remove from the oven and sit for 10 minutes or so before slicing into chunky pieces. This is also good at room temperature.