Boil crab apples and sugar in a pan with enough water to cover until soft.
Strain the liquid into a jug and stir in two gelatine leaves for every 500ml.
Strain again and chill the jelly in the fridge until set.
Mix rice, demerara sugar, and tea leaves in a bowl for the smoking mixture.
Line a wok with foil, place over high heat, and add the smoking mixture.
Season mackerel and place skin-side down on a greased cooling rack.
Place the rack over the smoking wok and cover tightly with aluminium foil.
Turn off the heat and allow the fish to cook slowly as it cools.
Serve the cooked mackerel fillets with the chilled crab apple jelly.