Pre-heat the oven to low 110°C, 225°F, Gas Mark ¼. Combine the brown sugar, vinegar and 75ml of water in a saucepan, dissolve over a low heat then allow to cool. Peel and core ½ of the apples and slice into thin rounds. Lay the slices of apple in a tray, brush with ½ the lemon juice and then coat liberally with the sugar and vinegar syrup. Place in the oven for 1 ½ hours until translucent and crystalline. Transfer to a wire rack to cool.
To make the jam, peel, core and chop the remaining apples roughly, then place in a saucepan with the remaining water, lemon zest and 1 tsp Rosemary, stir in the caster sugar and boil until the apple is puréed and setting point is reached. Allow to cool.
Toast the pine kernels under a hot grill until golden brown. Gently warm the olive oil with the remaining Rosemary, add the pine kernels and allow to cool and infuse for 30 minutes.
Sprinkle the Sea Salt flakes over the surface of a large non stick frying pan and place on a high heat. Place the mackerel fillets skin side down on the hot salt and press down on them to stop the fillets curling. Turn the heat down slightly and cook the fillets on their skin side until you can see the heat has penetrated half way through the fillet. Turn the fillets at this point to seal the flesh side very briefly. The mackerel should remain pink and undercooked in the middle.
To serve place two mackerel fillets per person skin side up on a serving plate. Place 4 slices of apple on top and serve with the apple jam on the side, spoon the pine kernels and rosemary oil around the fish to finish.