You will need to begin this recipe one day ahead.
Make a cure for the tuna by combining the salt, sugar and lemon zest in a small bowl. Rub the mixture vigorously into the fish and set aside to cure for 45 minutes. Wipe the cure from the tuna.
To smoke the tuna, line the base of a large, heavy-based saucepan with the hickory wood chips. Ignite them until you see them start to smoke then place the lid on top to capture the smoke. Working quickly, place a greased wire rack over the top of the wood chips, add the tuna and close the lid again. Set aside for 10 minutes for the smoke flavours to infuse the fish. Remove from the saucepan and put the tuna in a dehydrator overnight, or dry in a 50°C (120°F) oven. The aim is to dry the tuna to a jerky consistency that is firm enough to grate.
Put the 4 scrubbed potatoes in a large saucepan, cover generously with water and bring to the boil over high heat. Reduce the heat to medium and simmer for 20 minutes or until the potatoes are cooked through. Drain and set aside.
Make a beurre noisette by heating a frying pan over medium heat until hot, add the butter and cook, swirling constantly, until it turns a nut-brown colour. Set aside.
To make the potato espuma, boil the extra potatoes with the skin on for 20 minutes or until cooked through. Strain and set aside. When cool enough to handle, peel the potatoes. Heat the cream and butter in a saucepan over medium heat until the butter has melted. Add the xanthan gum and iota carrageenan to the heated cream, stirring to combine. Place the potato, cream mixture, chicken stock and salt in a blender and blend until smooth. Pass through a sieve into a pouring jug and keep warm until ready to serve.
To serve, warm the 4 whole potatoes then break them into bite-size chunks. Add the beurre noisette, season generously with salt and pepper and toss gently to combine. Boil the bone marrow for 1 minute in a saucepan of boiling water placed over medium heat. Strain and add the marrow to the buttered potatoes before scooping into serving bowls. Pour the puréed potato mixture into an espuma gun loaded with two chargers until it is three-quarters full, being careful not to overfill the gun. Shake the espuma gun and trigger potato foam over the top of the potato and marrow mix. Sift ground coffee over the top of the foam and grate about
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