Preheat the oven to 180 degrees C. Place the pork, skinned-side down, on a chopping board. Slit the joint lengthways, cutting two-thirds of the way through the meat, then open it out like a book.
Peel, core and finely chop 1 of the apples, then mix with the onion, prunes, biscuits, egg yolk and pepper. Spoon onto the pork, spreading evenly, then press the joint back together. Tie into a neat shape with string.
Put the joint, skinned-side up, into a roasting tin and pour in the stock and wine. Cover the tin with foil, twisting the ends tightly over the edges. Put into the oven and roast for 2 hours.
After 45 minutes, heat the oil in a second roasting tin on the shelf above the pork for 5 minutes. Add the parsnips to the tin and put in the oven to roast, turning them once or twice.
Meanwhile, make the apple sauce. Put the apples, ginger, sugar and water into a small saucepan. Cover and simmer, stirring occasionally, for 10 minutes or until pulpy. Remove from the heat and set aside.
When the pork has been cooking for 1 1/4 hours, remove the foil and drizzle the meat with the honey. Add the shallots to the parsnips, toss and continue roasting.
About 20 minutes before the end of the cooking time, peel, core and thickly slice the remaining 2 apples, then add them to the parsnips.
Transfer the pork to a carving board, cover with foil and keep warm. Strain the cooking liquid into a saucepan, then skim off any fat. Boil the cooking liquid, stirring constantly, for 2 minutes. Pour into a sauce boat. Carve the pork and serve immediately, with the roasted parsnips, apple sauce and gravy.