Preheat the oven to gas 6, 220C, fan 200°C With a sharp knife, score the fat side of the meat in a criss-cross pattern. Mix most of the fennel seeds with the lemon zest, garlic, parsley and season. Spread over the non-fat side of the meat, roll tightly and tie.
Put the meat on a rack in a roasting tin and sprinkle with plenty of salt. Cook for 30 minutes, then turn the heat down to gas 4, 180C, fan 160°C Pour 250ml (8 fl oz) of water into the bottom of the pan (this steams the meat and keeps it moist). Sprinkle the top with the rest of the fennel seeds and cook for a further 2 hours. It should be tender at this point, but if not, cook for another 20 minutes.
Mix all of the ingredients for the salsa in a small bowl. Slice the pork thickly and erve with the salsa and gratin.