Preheat the oven to 220°C conventional oven/ 200°C fan-forced (425°F/gas 7).
Use a sharp knife to score the skin of the pork belly diagonally in a criss-cross pattern, without cutting into the meat.
Place the fennel seeds into a mortar and pestle and crush. Stir in the chopped rosemary and olive oil.
Place the pork on a chopping board. Use paper towel to pat the pork skin. Season the pork generously with salt and pepper. Use your hands to rub the salt and pepper into the skin. Place the pork skin side down. Rub the olive oil, crushed fennel and rosemary into the sides and bottom of the pork (being careful not to rub any olive oil over the skin). Roll the pork up tightly and tie with kitchen string.
Place the pork, rind side up into a baking tray and roast for 20 minutes.
Reduce the heat to 160°C conventional oven/ 140°C fan-forced (325° F/Gas 3). Cook for a further 2 hours.
Remove the pork from the oven. The skin should be golden and crunchy while the meat of the pork lovely and moist. Transfer the pork to a chopping board and allow to rest, uncovered for 30 minutes before carving – buon appetito.