Mix the thyme, mace, star anise and sea salt flakes together in a small bowl. Rub all over the pork belly. If you have time, transfer the pork to a tray and marinate in the fridge for up to 24 hrs, but this is not essential.
Heat the oven to 220C/200C fan/gas 7 and butter a large roasting tin. Cut a thin slice lengthways from both sides of each potato, then sit them flat in the prepared tin in one even layer. Scatter over the thyme and onions, then pour over the chicken stock.
Sit the pork on top of the potatoes, skin-side up and roast for 30 mins. Reduce the oven to 180C/160C fan/gas 4 for 1 hr 30 mins, until the skin is crisp. Transfer the pork to a board, then cover loosely with foil and leave to rest for 20 mins.
Return the tin with the potatoes to the oven and increase the oven temperature to 200C/180C fan/gas 6 for 25-30 mins until crisp. Carve the pork into generous squares (you can use a bread knife to do this more easily) and serve alongside the fondant potatoes and caramelised pickled onions.