Pre-heat the oven to 200 degrees. Whisk the egg whites until they start foaming up. Take the salt out into a large bowl and then pour the egg white mix into the salt.
Use your hands to mix the salt and egg whites and have a feel of the consistency.
The salt dough should feel like wet sand. Add a few spoons of cold water if the salt dough feels too dry.
Rinse the fish under cold water and pat dry with a kitchen towel. Cut the fins from the fish with a sharp kitchen scissor. Stuff the cavity with the parsley stems (if you are making the salsa verde), crushed garlic, lemon thyme and sliced lemons.
On a large baking tray spread one third of the salt dough and press down with the palm of your hands to have an even layer. Place the fish on top and cover with the remaining salt dough. Stroke the salt dough with wet hands from head to tail making sure there are no cracks forming on the surface.
Place the fish in the oven and cook for 40 minutes. To check the doneness stick a metal skewer through the salt crust into the fish. If the skewer feels hot the fish is done, if the skewer is only warm keep cooking for another 5 minutes.
Remove the fish and let it rest for 5-10 minutes. Take a sharp knife and run the tip of the knife around the crust where you will make the cut.
Now start cutting into the crust along the line until you are able to remove the top part.
Use a cake lifter to serve the fish fillets.