Preheat the oven to 180°C.
Line a roasting tray with greaseproof paper. Score the skin of the pork belly quite deeply (around 1cm), then place it skin side down on the paper-lined tray. Season the top of the pork belly with salt, pepper and cumin seeds and cook for 1½–2 hours. The skin should be very crispy and the meat must be tender – if it’s not quite there yet, turn it over and cook it for another 10 minutes.
Make the mojo verde while the pork belly is cooking: roughly chop the spring onions, garlic and herbs and add them gradually to a pestle and mortar with the rest of the ingredients, dribbling in the olive oil bit by bit and mashing together.
Spoon some mojo verde on to each plate, then top with 1–2cm thick pork belly slices and drizzle over a little olive oil to finish.