Begin by preparing the leeks for both the broth and potato mash. Preheat a grill to high then char the leeks on all sides for a few minutes to soften (this can also be done on a barbecue if preferred)

Preheat a water bath to 85°C

Discard any blackened edges and measure out 60g for the broth. Place in a vacuum bag with the water and a pinch of salt, then seal in a chamber sealer and cook in the water bath for 3 hours

Cut the remaining leeks for the potato mash into thin slices and set aside until needed

Meanwhile, prepare the mackerel by cutting into fillets. Remove the head and run the knife down the length of the fish, keeping as close to the backbone as possible. Repeat on the other side and remove the bones, reserving these for the smoked oil

Use tweezers to remove any pin-bones from the fillets and cut each piece into 4 diamond shapes. Reserve in the fridge until ready to cook

Preheat a grill or barbecue. Wash the bones well to remove any traces of blood. Place on the hot grill for 10 minutes to char the bones slightly, then transfer to a smoker and allow to smoke for 45 minutes

Preheat the oven to 165°C/gas mark 3

Meanwhile, place the potatoes for the mash on a baking tray and bake in the preheated oven for 45 minutes, or until tender

Once the bones have finished smoking, place the extra virgin olive oil in a large pan and bring up 180°C over a medium-high heat. Carefully place the bones in the oil, remove the pan from the heat and allow to infuse until completely cool

Remove the baked potatoes from the oven. Once cool enough to handle, peel the potatoes and mash until smooth, then stir through the reserved charred leek slices and season with salt. Set aside until serving

Preheat the oven to 140°C/gas mark 1

Strain the smoked mackerel oil through a fine sieve and discard the bones. Weigh out 8g of the oil and combine in a bowl with the maltodextrin and a pinch of salt, mixing to form a white powder

Place the resulting powder on a sheet of baking paper and cover with a second sheet. Roll out into a 5mm thick sheet, place on a baking tray and bake for 10 minutes until set. Allow to cool then roughly chop into shards and reserve until serving

Mix together the vinegar, salt and water for the marinated mackerel. Place the fillet portions in the marinade and allow to sit for 8 minutes. Drain from the marinade, then cover the mackerel with the extra virgin olive oil

Once the leeks have finished cooking in the water bath, pour the contents of the bag through a sieve into a pan, discarding the leeks. Bring the broth to the boil over a medium heat and add the bonito flakes

Remove the pan from the heat and quickly cool in a large bowl iced water – this will help keep the broth clear. Pass the cooled broth through a sieve again to remove the bonito flakes and keep warm in a clean pan until serving

To serve, place the potato and leek mash in a pan with the olive oil and warm through on the hob, beating to keep it smooth. Place a small rectangular mould in each serving dish and spoon in the mash, pressing down to create an even layer before removing the mould

Drain any excess oil from the mackerel fillets and divide between the plates, placing on top of the potato. Lightly scorch the edges of the fish with a blowtorch to blacken slightly

Drizzle a little of the remaining smoked mackerel oil over each dish and sprinkle over a little salt to season. Pour in some of the hot broth and garnish with the reserved smoked mackerel shards and pea shoots to serve