Salt-baked fish is a whole fish covered in a thick crust of coarse salt and baked. The crust hardens in the oven and forms a sealed chamber; the fish cooks in its own steam, stays moist, and is surprisingly not too salty. The fish itself is neither salted nor scaled, because its skin and scales act as a barrier between the salt and the flesh. The crust is cracked open at the table. The method looks impressive, but it is simple to do.
The salt crust does two things. First, it insulates: the thick layer of salt softens the oven’s dry heat and transfers heat to the fish evenly and slowly, which is why salt-baked fish does not dry out. Second, it seals the fish in; once the crust hardens, steam cannot escape, and the fish cooks in its own moisture. Less salt reaches the flesh than you might expect because the skin and scales form a barrier. That is why you do not salt the fish separately before baking it in salt.
Use the fish whole and remove its innards, but leave the scales on; the scales provide another shield against the salt. Put a lemon slice, bay leaf, fresh thyme, or parsley in the cavity to flavor the fish from within. Do not rub salt or seasoning on the outside. You can leave the tail and head uncovered. Pat the fish dry, because a wet surface makes the salt stick to the flesh and can leave the finished dish much saltier than expected.
Use coarse sea salt for the crust; fine table salt is unsuitable because it does not form a firm crust and can make the result much saltier. Mix the salt with a little water or beaten egg white until it has the texture of wet sand so it will hold together. Use a generous amount: you need at least twice the fish’s weight in salt. Spread a bed of salt on the baking sheet, set the fish on top, and cover it completely with the remaining salt, leaving no exposed spots.
Preheat the oven and set it to about 200°C / 390°F. Cooking time depends on the fish’s weight; as a rough guide, allow 25 minutes per kilogram, with less time for individual portions. As it bakes, the crust hardens, cracks slightly on top, and darkens a little. When the time is up, remove the baking sheet from the oven and let the fish rest inside the crust for a few minutes. The measure is still the internal temperature: insert a thermometer through a hole in the crust and look for 63°C / 145°F. Since you cannot see the fish beneath the crust, do not extend the time based on guesswork; if it is undercooked, you can return it to the oven after opening the crust.
Crack the crust at the table; this is the most dramatic part of serving. Tap the top crust with the back of a knife or the handle of a spoon, lift it along the crack, and carefully remove the pieces. Once the crust is lifted, peel away the skin with the salt to reveal moist, unsalted flesh. Remove the fillets with a spoon, keeping salt crystals off the meat. Olive oil and lemon are enough alongside; a heavy sauce would mask the lightness of this method. Do not reuse the remaining salt crust; discard the salt that has become wet and absorbed fish juices, and wash the baking sheet right away.