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1
Heat the oven to 200 °C. Combine the coarse sea salt, lightly beaten egg whites, and a small amount of water in a large bowl. Work the mixture with your hands until it clumps firmly when pressed — it should hold its shape but not be wet. Test: a handful squeezed together holds a clean impression.
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2
Spread a 1 cm layer of the salt mixture across the base of a baking dish large enough to hold the fish flat. Stuff the cavity with thyme, lemon zest, and bay leaves if using. Lay the fish on the salt bed.
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3
This is where the dish lives or dies: cover the fish completely with the remaining salt mixture, pressing firmly on all sides to form a smooth, unbroken crust with no gaps or cracks. Run your palm over every surface. Any gap will allow steam to escape, drying the flesh beneath. The crust must be fully sealed and of even thickness — sight-check from every angle before the dish enters the oven.
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4
Bake at 200 °C for 20–25 minutes per kilogram of fish. A 700 g fish requires approximately 17–18 minutes; err toward the longer estimate, as the dense crust insulates. The crust will be set, dry, and very lightly coloured on the surface. Remove from the oven and rest 5 minutes outside the oven before cracking.
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5
At table, strike the crust firmly along its crown with a mallet or the handle of a heavy knife until it fractures. Peel the crust away in sections. Using a pastry brush, sweep any remaining fine salt crystals from the skin before filleting — residual crystals on the flesh will over-season. The flesh beneath should be white, opaque, and fragrant. Lift the fillets directly from the skin, which is not served.