1. Score the skin of the mackerel fillets at 2cm intervals and set aside.
2. For the marinade, mix all the remaining ingredients together in a small bowl.
3. Brush the flesh side of the mackerel fillets with the marinade. Cover and leave to marinate in the fridge for 6–12 hours. This will cut some of the richness of the mackerel and also sharpen the flavour. Remove the mackerel fillets from the fridge half an hour before cooking.
Note: Putting the fish to marinate a day in advance allows the flavours to develop and penetrate the fish.
4. Put the apple juice into a bowl. Finely grate the ginger and place in a small bowl. Pour on the water and leave to soak for 1 minute. Strain the ginger water through a fine sieve into the apple juice, pressing the ginger to extract as much flavour as possible; discard the ginger. Put the apple pieces into a small sous-vide bag and add the ginger apple juice. Vacuum-pack on full pressure. Open the bag, then repeat the process. This will force both flavour and juice into the apple, changing its texture and taste completely – the apple will become almost translucent. Set aside until needed.
5. Using a mandoline, cut the mouli into ribbons, 8cm long and 2mm thick.
6. Tie the ginger, Szechuan pepper and lime zest in a piece of muslin and place in a large saucepan with all the other ingredients except the mouli. Bring to the boil over a high heat, then lower the heat and simmer for 2 minutes. Add the mouli ribbons, return to a simmer and cook for 1 minute. Remove from the heat, let cool, then refrigerate the mouli in its cooking liquor until ready to serve.
7. Peel, quarter and core the apples. Cut into 5cm pieces and place in a sous-vide bag with the ginger. Vacuum-pack on full pressure then cook in a steamer or large pan of simmering water for 8–10 minutes until soft. Remove from the bag and drain, reserving the juices. Puree the cooked apples and ginger in a blender until smooth, adding some of the reserved juice to achieve the desired consistency. Taste and add a pinch of sugar or dash of lemon juice if required. Pass the puree through a fine sieve into a bowl and refrigerate until ready to serve.
8. Put all the ingredients except the gelatine into a small saucepan and warm gently over a low heat to 60–70°C; maintain the mixture at that temperature for 5 minutes; do not allow to boil. Taste and adjust the seasoning, adding more soy, sugar and/or lime juice as required. Strain the liquor through a fine sieve into a small saucepan. Pour 75g of the hot liquor into a warmed small bowl. Immediately drain the gelatine and add to this liquor, stirring until dissolved. Pour the mixture into a small shallow container and leave to set in the fridge to a light and flavourful jelly. Bring the remaining liquor (about 80g) to a fast boil and let it bubble to reduce it by half to a light glaze. Reserve for glazing the mackerel.
9. Put the watercress into a large bowl. For the dressing, whisk all the remaining ingredients together in another bowl to emulsify, then taste and correct the seasoning if necessary. Just before serving, pour the dressing over the watercress and toss to distribute.
10. Preheat the grill to high. Brush the skin side of the mackerel fillets with the glaze and leave to stand for 10 minutes. Repeat the brushing, to create a thick glaze.
11. Lay the mackerel fillets skin side up on a lightly oiled tray and grill for 4–5 minutes according to their size, until the skin is crisp and an appetising dark amber colour. Finish with a squeeze of lime juice and a pinch of sea salt. Place the drained mouli ribbons in the centre of each plate and arrange the pieces of compressed apple and watercress around them. Place 3 slices of jelly and a spoonful of apple and ginger puree on each plate. Finally, lay the grilled mackerel fillets on top of the mouli and serve.