Pork belly is a reasonably priced cut of meat, but it does need a long, slow roast to bring out the best in it. The anise notes of the fennel seeds complement the pork perfectly.
Preheat your oven to 230°C (446°F). Deeply score parallel lines in the fat, about 1cm (half an inch) apart. Use a Stanley knife or box cutter for this, or ask your butcher to do it if you can't manage it. Crush the fennel seeds, and mix them together with the salt. Rub the mixture right into the cuts in the pork fat, and rub the remainder into the underside of the pork.
Chop the carrot in half lengthways, and then into thirds. Cut the celery into thirds. Slice the onion into 1cm (half inch) slices. Lay these in a single layer in a baking tray and place the pork on top of this platform. The veggies are there to enhance the flavour of the cooking jus. Pour in enough boiling water to cover the veggies, and place in the hot oven for 30 minutes.
After 30 minutes, check the pork to make sure that the skin has started to blister all over - if it hasn't, give it a bit more time at the high temperature. If it has, turn down the temperature to 140°C (284°F) and cook the meat for 3 hours.
When the meat is cooked, remove it from the oven and let it rest, tented in foil, for 20-30 minutes. To make a gravy, strain the pan juices into a saucepan (discard the remains of the vegetables), stir in a tablespoon of flour, whisking rapidly to prevent lumps forming, and simmer for a few minutes. Taste for seasoning, and add salt if necessary.