Score the skin of the pork belly at 5mm intervals.
Pat pork completely dry with paper towel. Place pork skin side down onto a chopping board. Cut vertically into the thickest park of the pork along the long side, three-quarters of the way through, to form a flap. Season well.
Place the sage, rosemary, thyme, lemon zest, garlic, and oil into a blender and blitz until a paste consistency. Press this mixture into the centre of the pork, roll up firmly from the long side. Tie tightly at 4cm intervals with kitchen string. Gently press any loose stuffing back into the ends.
Transfer to a large plate. Refrigerate overnight, uncovered for the skin to dry out.
Preheat oven to 240° C/220°C fan-forced. Transfer pork to a wire rack on a baking tray. Bring to room temperature.
Roast pork for 30-35 minutes until the skin is golden and crispy. Reduce heat to 180°C/160°C fan-forced and roast for 1-1.5 hours. Add the grapes to the tray around the pork for the last 30 minutes. The pork is ready when the internal temperature reaches 60°C, or until the juices run clean when tested with a skewer in the centre. Loosely cover with foil and rest for 15 minutes.
Carve the porchetta and serve with roasted grapes.
