Preheat the oven to 120C/100C Fan/Gas ½.
For the pork belly, in a medium lidded casserole, mix all the ingredients except the pork. Add the belly and baste with the sauce. Bake for 3 hours, basting with the sauce every 20 minutes. When cooked, the pork will be firm but offer a little resistance when a knife is inserted. Lift the pork out of the casserole and set aside to rest, loosely covered with foil.
Strain the cooking juices into a saucepan over a high heat and bring to the boil. Reduce the volume of liquid by half so it thickly coats the back of a spoon. Remove from the heat and reserve.
For the kale, in a large lidded saucepan, mix together 100ml/3½fl oz water, the butter and salt and bring to the boil. Add both varieties of kale and cook, covered with a lid, for 3–4 minutes, until barely cooked but still retaining a bit of bite and their wonderful colour. Drain well.
To serve, spread a spoonful of the quince purée in the centre of each plate. Add the kale, then slice the pork belly and divide between the plates. Finish with a little more purée and scatter over the sesame seeds.