Preheat the oven to 200C/400F/Gas 6.
For the pork, place the walnuts, apples, thyme and two-thirds of the butter into a bowl, season with salt and freshly ground black pepper and mix to make a chunky paste.
Lay the pork, flesh-side up, on a chopping board with the belly flap facing away from you. With a sharp knife, make an incision in the meat halfway down running parallel to the board, cutting almost all the way through. Open the loin up, stuff with the butter mixture and season with salt and freshly ground black pepper.
Generously grease a piece of foil large enough to wrap the pork and place the pork on top. Rub the remaining butter all over the pork, then roll up in the foil. Twist the ends of the foil so that the parcel is very tight and tie with some string.
Heat a large roasting tin on the hob until hot and add the vegetable oil. Place the foil parcel into the tin and cook, turning every 2-3 minutes, until all parts of the parcel have had contact with the heat - this will colour the outer layer of the meat.
Transfer the pork to the oven and roast for 1-1½ hours, or until golden-brown and completely cooked through.
Meanwhile, for the parsnips, place the parsnips into a roasting tin. Drizzle over the olive oil and honey and stir well to coat the parsnips. Season with salt and freshly ground black pepper, add the thyme sprigs and mix well.
Place into the oven and roast for 35-40 minutes, or until tender and golden-brown.
Add the walnut halves and return to the oven for five minutes, until lightly toasted.
To serve, unwrap the pork parcel, discarding the foil, and slice the pork into thick slices. Place the slices onto six plates with the parsnips and a drizzle of gravy.
