1. To brine fish, bring the salt and water to the boil and whisk until dissolved, cool down liquid and submerge fish for an hour, once out of the brine, pat dry and cut into 2 pieces, set aside until ready to cook.
2. For champagne sauce, start by sweating down all the vegetables and spices in a little bit of oil, you want no colour and to bring out all the sweetness of the veg. Once that's done pour in the champagne and begin to reduce by half, add in the fish stock and reduce by ?. Add in the cream and reduce again by half. Strain through a fine sieve and set aside, discard the veg.
3. To prepare the creamed leek and mussel mix, in a wide pan melt the butter with a dash of oil, add in leeks and stew down gently to release the sweetness and vibrant colour, season with salt and pepper and add the cream, cook out for 2 minutes, remove mix for the pan and set aside. With the mussel meat, cut each piece in half and place 20 cut pieces in a small saucepan, add 5 tablespoons of leek mix and set aside
4. To cook the fish, heat up all the oil in a wide saucepan and heat 65 - 70 degrees, place both pieces of fish gently in the warm oil and poach until a skewer penetrates the fish with no resistance. Remove pan from the heat and sit the fish in the oil until you're ready to plate
5. Warm your leek and mussel mixture gently until it comes to temperature and sit two spoons worth on the bottom of your bowl, place fish on top of the leek, sitting up. Place the caviar on top of the fish with the slices of truffle. To warm the sauce, place some in a small saucepan, heat until it is hot to touch and with a stick blender, aerate on an angle to form lots of bubbles. Sauce around the fish and present.