For the jelly, put the apples and sugar in a pan and pour in enough water to cover. Bring to a boil and cook until the apples are soft and falling apart.
Strain the cooked apples through a sieve and pour the strained liquid into a measuring jug. For every 500ml/18fl oz of liquid you will need two leaves of gelatine. Add the gelatine to the liquid while it is still hot, stir to dissolve, strain again then chill the jelly in the fridge.
For the smoking mixture, mix together the rice, demerara sugar and tea leaves in a bowl. Line a deep pan or wok with aluminium foil, place over a high heat and add the smoking mixture.
Season the mackerel with salt and freshly ground black pepper. Grease a cooling rack with a little oil and place the mackerel on the rack, skin side down. When the mixture begins to smoke, place the rack on top of the pan or wok and cover with aluminium foil. Turn off the heat and let the mackerel cook slowly as it cools.
When cooked, place the mackerel fillets onto serving plates and serve with the crab apple jelly.