Begin with making the salt dough, by whipping up the egg whites to stiff peaks. Roughly chop the thyme leaves and add to the salt. Now add the egg whites to the salt to make a paste that resembles a wet sand texture. Lay baking parchment on a tray and pat down some of the salt dough mix to form a bed for the fish, then completely cover the fish with this paste and pat down firmly. Place this in a hot oven at 170c and bake for 20 minutes. In the meantime chop all the herbs together and mix. Chop the capers and gherkins and add to the herbs, along with the anchovies - then mix the mustard, vinegar and olive oil. This will produce a vibrant green salsa and ready to spoon over the salt baked hake. Crack open the crust and remove the fish, making sure not to serve any of the salt crust. Spoon over the salsa and serve.