To make the master pork broth, blacken the cut side of the onions in a very hot, dry pan. Place in a large pan with all the other ingredients and 5 litres/8 ¾ pints of water. Bring to a gentle simmer. Cook for 3 hours, skimming every 30 minutes or so. The trick here is to simmer at a bare roll and skim any impurities regularly, to achieve a clean, clear broth.
Without moving the pan, turn off the heat and gently ladle the broth out of the pot through a very fine sieve, taking care not to disturb the bottom too much. Cool and reserve the master pork broth until needed. (You can use the meats and vegetables to make pork broth salad or pork broth terrine.)
For the pork, generously salt the pork belly and leave uncovered for 1 hour, then rinse. Place in a large saucepan and cover with the pork broth. Add the rest of the ingredients.
Bring to the boil, then turn down to a simmer. Place a plate or a small saucepan lid directly on the pork belly to keep it submerged. Cook at the barest simmer for about 2 hours. When you can pass a skewer through the belly with ease, it is ready.
Meanwhile preheat the oven to 220C/200C Fan/Gas 7.
Take care when removing the pork belly from the stock. Place it on a chopping board and remove the bones and sinews (they should pop out of the tender meat).
Score the skin slightly in a chequered pattern. Place on greaseproof paper on a baking tray and roast for about 20 minutes.
Meanwhile, strain the reserved stock into a saucepan and reduce it until a nice coating consistency is achieved, like a light syrup.
For the dressing, whisk all the dressing ingredients together until the sugar has dissolved, and store in a sealed container.
For the slaw, toast the fennel seeds in a dry pan until golden-brown, then set aside.
Slice the cabbage, fennel and onion 1 mm thick, using a mandolin – the thinner the better. Finely chop the herbs using a very sharp knife.
Toss together with all the remaining ingredients so that they are well combined and the herbs are evenly spread throughout the slaw.
Whisk the dressing again 20 minutes before serving and add to the slaw. Toss thoroughly and check and adjust the seasoning.
Serve the pork with the sauce drizzled over it and the slaw on the side.