Cut the pork belly into pieces about 2 cm–5 mm (¾–¼ inch). Heat a frying pan over a high heat and cook the pork for 4 minutes until golden and crispy, then drain on kitchen paper.
Make the dressing. Put the sugar in a small, heavy-based saucepan and cook for 4 minutes over a medium heat until the sugar is bubbling and has turned a deep caramel colour. Carefully, because it might spit, whisk in the lime juice, fish sauce and water. Remove from the heat and set aside to cool slightly.
Combine the pomelo segments, the prawns, the herbs and the cooked pork belly in a large salad bowl. Toss lightly and transfer the salad to serving plates. Drizzle the caramel dressing over the top, garnish with roasted peanuts and serve.