Using a really sharp knife (or a Stanley knife), score the pork belly's skin horizontally, in 1½ cm intervals. Flip the belly over & lightly score the meat in a criss cross pattern. Place the meat in the fridge whilst you make the marinade.Make sure not to score the skin too deeply, you don't want to cut into the meat! The skin will be easier to score if the pork is cold, so I'd recommend scoring the belly straight from the fridge. Place the fennel seeds into a small frying pan then toast over a medium-low heat, until fragrant. This will take a minute or two.
Transfer the toasted seeds into a pestle & mortar then grind into a coarse powder. Next, grate in the garlic & add in the zests, chopped rosemary, olive oil, salt & plenty of freshly cracked black pepper. Stir to combine.
Next, add the nduja into a food processor then give it a quick blitz, to make it more spreadable. This will only take 30 seconds or so.
Take your pork belly out of the fridge & place onto a chopping board, skin side down. Spoon over the marinade then massage it into the meat. Add the nduja on top then spread all over the meat, in an even layer.
Cut 8-10 lengths of butcher's twine & set to one side. The twine needs to be long enough to comfortably wrap around the pork belly.
Next, roll the pork belly up, into a tight log then place seam side down on your chopping board. Tie the belly up with the twine, leaving a couple of centimetres between each piece. To avoid losing any of the filling, tie a piece of twine at each end then work towards the centre.
Once prepped, place the porchetta onto a baking tray & leave in the fridge overnight, uncovered. This is key to getting the best crackling!