Sauté the fish loins in salted butter, giving each fillet a golden brown color. Remove prawns from shell, and sauté lightly. Reserve fillets and prawns.
To prepare sauce, bring fish stock, wine, shallots, bay leaf, peppercorns and lemon juice to a boil, and reduce to 150 milliliters of liquid and strain. Add the nuts to the beurre blanc base; blend it with a hand blender until very smooth. Add cold butter and heavy cream, and blend again. Do not heat up again, as the beurre blanc will lose its emulsion. Keep in warm area until serving.
To prepare vanilla air, bring 800 milliliters of water to boil and infuse with vanilla beans for 6 hours. Add 10 grams of Lecite per liter to the vanilla water.
To prepare lemon gelée, bring sparkling water to a boil, and add the agar agar and the gelatin sheet. Mix in lemon juice and syrup, and pour it into a rectangular pastry baking pan lined with plastic wrap. Refrigerate. Once solidified, cut the gelatin into cubes.
Boil potato (with skin still on) in salt water. Peel and mash with a fork. Add olive oil and salt. On a plate, stack potato “Trinxat” and two fillets, drizzle with beurre blanc and top with small serving of vanilla air. Place two prawns and two warm lemon gelée cubes on plate.