Preheat the oven to 200C/400F/Gas 6.
Place the pork, flesh side up, onto a chopping board with the belly flap away from you. Using a sharp knife make an incision into the pork halfway down, running parallel to the board, cutting almost all the way through. Open this pocket in the pork loin and season inside with salt and freshly ground black pepper.
Place the fresh herbs into a bowl with the lemon zest and the olive oil and mix well.
Rub the herb mixture onto the pork quite forcefully, then sprinkle over the breadcrumbs - this will help to distribute the flavours evenly.
Roll up the pork loin tightly and leave to sit for 20 minutes.
Cut a piece of aluminium foil large enough to wrap the pork and lay flat. Place the slices of pancetta evenly across the foil, overlapping slightly. Place the pork on top and roll up the foil, pancetta and pork loin. Twist the ends so the parcel is very tight like a sausage.
Heat a large roasting tray over a high heat until hot, then add the vegetable oil. Place the foil parcel into the tray and cook, turning every two minutes, so all parts of the parcel have made contact with the hot oil - this will colour the outside of the meat.
Transfer to the oven and roast for 1-1½ hours, or until completely cooked through. To test it is cooked, insert a metal skewer through the foil and into the centre of the meat - if the juice runs clear when the skewer is removed the pork is cooked. Remove from the oven and leave to rest for five minutes, then gently unwrap and cut into slices.
For the parsnip purée, place the parsnips and potatoes into a saucepan with the milk. Add enough water to just cover the vegetables, season well with salt and freshly ground black pepper and bring to the boil. Reduce the heat and simmer for 20 minutes, or until the potatoes and parsnips have become soft.
Drain most of the liquid off from the pan and add the butter and crème fraîche. Using a stick blender or a food processor, blend the parsnips and potato mixture until smooth, then season, to taste, with salt and freshly ground black pepper.
To serve, place a couple of slices of the pork onto warm plates with a spoonful of the parsnip purée on each.