Preheat the oven to 220°C and lightly oil a deep roasting tin.
Fill the fish cavity with lemon slices and season with pepper.
Mix herbs, sea salt, lemon zest, chilli flakes, and egg white in a bowl.
Layer one third of the salt in the tin, place fish on top, and cover with remaining salt.
Bake the fish for 35 to 40 minutes.
Boil vinegar and water in a saucepan until reduced to 5ml.
Cool the reduction, then whisk in egg yolks over low heat until smooth.
Gradually whisk in 225g of butter until thick, adding warm water if necessary.
Melt 15g of butter in a pan and wilt the sorrel leaves.
Chop the wilted sorrel and stir it into the warm hollandaise.
Remove fish from the oven and carefully crack open the salt crust.
Serve the fish on a platter accompanied by the sorrel hollandaise.