The night before, boil a kettle and pour the boiling water over the pork skin (this method will give you better crackling). Discard the water.
Next, baste the raw joint with cider vinegar; rubbing it all over to ensure the vinegar gets into the rind. Place the joint on a plate in the fridge, uncovered, overnight.
Remove the pork from the fridge and spread with the dripping or butter, crushed bay leaves and garlic. Season with salt and freshly ground black pepper, set aside for the flavours to develop.
Pre-heat the oven to 200C/400F/Gas 6.
Place the pork ,skin side up, on a wire rack that fits over an empty roasting tray. Roast for one hour and then reduce the oven temperature to 180C/350F/Gas 4.
Place the parsnips, carrots and shallots into the bottom of a roasting tray; add the thyme and olive oil and a glass of water.
Remove the tray from beneath the pork and pour any fat that has gathered in the bottom of the tray over the vegetables; place the vegetables tray underneath the pork.
Roast for a further hour, basting the vegetables with the juices from the pork in the bottom of the pan from time to time.
Remove the pork from the wire rack and allow to rest for 15 minutes in a warm place.
Toss the spinach leaves with the vegetables from the roasting pan. Drain off any fat leaving the cooking juices behind.
Carve off the crackling in one piece and portion. Serve the pork with the roasted root vegetables and gravy.