To make the brine, place the salt and 3 litres of water in a large saucepan, stirring until salt dissolves. Cool to room temperature, then transfer to a 4 litre plastic container (large enough to hold the pork belly) and place in the fridge to chill.
Immerse the pork belly in the chilled brine to cover completely. Seal the container and place in the fridge for 24 hours. An hour before roasting, remove the pork belly, discarding the brine. Pat dry all over with paper towel. Using a mortar and pestle or small food processor, pound or grind the salt and fennel seeds until the seeds are coarsely ground.
Preheat the oven to 220°C (200°C fan-forced). Score the pork at 5mm intervals using a sharp knife. Rub the salt mixture over the skin and into the cuts. Place the belly skin side up on a rack in a roasting tray.
Roast for 15 minutes, then reduce oven to 190°C (170°C fan-forced) and continue to roast for 40 minutes, until skin crackles and meat is just cooked through, the juices will run clear when a skewer is inserted. Rest for 15 minutes in a warm place.
Meanwhile prepare the salad. Place lemon zest, juice and oil in a screw-top jar and shake to mix well. Season to taste. Combine the apple, watercress and coriander in a serving bowl and pour over the dressing.
Carve the pork belly into 1cm wide slices and serve with the salad.