Preheat the oven to 180ºC, fan 160ºC, gas 4. Score the skin of the pork with a sharp knife, then make about 15 small incisions in the sides of the meat and stuff each with some of the chopped rosemary and a slice of garlic. Rub the skin with half the sea salt.
Heat 1 tablespoon of the oil in a large lidded casserole over a medium heat. Add the onions and fennel seeds and cook for 5 minutes, until the onion is just softened and golden.
Add the apples, cider, fennel, parsnips, carrots, bay leaves and the remaining rosemary. Pour in the stock, bring to a bubble and let simmer for 1-2 minutes, until reduced slightly.
Sit the pork in the vegetables, then cover and cook in the oven for 45 minutes. Take the roast out of the oven. Scatter the remaining salt over the pork skin, pressing it in with the back of a metal spoon.
Increase the oven temperature to 220ºC, fan 200ºC, gas 7. Put the potatoes in a bowl and coat with the remaining oil. Add them to the casserole and return it to the oven, uncovered, for a further 45 minutes.
Transfer the pork to a chopping board to carve. Serve 100g of pork per person along with pieces of the crackling and the vegetables, drizzled with a spoonful of the pan juice.
Cook's tip: for extra crispy crackling, move the pork joint to a baking tray on the top shelf of the oven for the final 20 minutes cooking time. Replace the lid on the casserole pot and move to the bottom shelf.