For really crisp crackling, rub the pork skin generously with the salt, put the pork skin-side down on a wire rack in a roasting pan, cover and leave in the refrigerator overnight to draw out excess moisture.
Preheat the oven to 475 degrees. Pat the pork dry with paper towels. Put skin-side up in a roasting pan. Drizzle a little olive oil over the skin, then rub in the fennel seeds and sea salt, making sure they go down between the score lines. Roast for 30 minutes.
Pour the lemon juice over the meat. Reduce the oven temperature to 315 degrees and cook for a further 1½–2 hours, checking twice during that time that the pork fat and juices are not burning on the base of the pan. If they are, just add a cup of water. At the end of the cooking time, the meat will be meltingly soft and the skin wonderfully crisp. Continue to roast the pork if you want the skin even crispier; it won’t do the meat any harm. Transfer the pork to a board, cover loosely with foil and leave to rest in a warm place for up to 30 minutes.
When you are almost ready to serve, throw together the warm salad. Heat the oil in a large frying pan over a medium heat, add the fennel and green beans and fry for about 8 minutes until just tender. Add the apples and chilli/hot pepper flakes and cook for about 2 minutes until just tender. Stir in the cannellini beans and stock. Bring to the boil, then reduce the heat and leave to simmer for 2 minutes until the beans are heated through. Season lightly with salt, pepper and a squeeze of lemon juice.
Cut the pork into large pieces, using a serrated knife, and serve with the warm bean salad.