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 aniseed notes of the fennel seeds complement the pork perfectly, and the cider in the gravy makes it sing.
Preheat your oven to 150°C (302°F) for a fan/convection oven, 170°C (338°F) for a conventional one, gas 3. Deeply score parallel lines in the fat, about 1cm (half an inch) apart. You want to penetrate the skin, and partially into the fat, but not into the meat. 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.
Slice the carrot, celery and 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 cider to cover the veggies. Tightly cover the roasting tin with foil and place in the oven for 3 hours. You might want to check the liquid level halfway through and top it up if necessary.
After 3 hours, turn up the oven to 180°C (356°F) fan, 200°C (392°F) conventional, gas 6. Remove the foil (don't throw it away) and let the meat cook for another 20 minutes. If you're doing roast potatoes, you'll want to pop those in now.
When the meat is cooked, remove it from the oven and move it onto a warmed plate. Let it rest, tented in foil, for 20-30 minutes. To make a gravy, mash the trivet vegetables, then press them through a fine sieve into a bowl. In a small saucepan, melt the butter and sift in the flour. Blend into a thick paste and cook for a couple of minutes. Add the pan juices, and the stock and bring to the boil. Taste for seasoning, add salt if necessary. Turn down the heat and keep warm until needed.
Now, the exciting part. When everything else is ready, make your crackling - you'll need a builder's blowtorch or an electric heat gun. Run these over the pork skin a few times to heat it up. When it's very hot, the skin will begin to bubble up - this is your amazing crackling! IF you don't have a heat gun or blowtorch, you can just put the joint under the grill or broiler, but it will take longer to crackle.