Remove the pin bones and scales from the hake. Cut into four equal portions then place in the fridge for later. For the crust, use a selection of wild herbs and some parsley. Pick the leaves from the stalks and discard the latter.
Remove the crusts from the slices of bread and dice the bread into 1cm cubes. Put the bread into an electric blender and mix on full power until it starts to turn sticky and hot. Add the herbs and a pinch of salt and continue to blend on full power for about 3 minutes, or until it is smooth and bright green.
Continue blending and add the butter a little at a time. Finally mix in the Parmesan. Taste the crust and adjust the seasoning if needed.
Spread the crust between two sheets of greaseproof paper into an even layer about 3mm thick then place in the freezer for 1 hour. Portion four pieces of herb crust to the same size as the portions of hake. Place in the fridge.
Remove the fish from the fridge 10 minutes before cooking. Heat the oven to 180C/Gas Mark 4 and turn on the grill.
To cook the hake, heat about 2 tablespoons of oil in a large, heavy-bottomed pan over a high heat. Pat the skin of the fish dry with kitchen paper, then season generously all over with sea salt. Place the fillets of hake into the pan, pressing down gently to ensure contact with the pan. Sear on the skin side for 1 minute over a high heat until golden, then place in the oven for 3 minutes. Remove from the oven and add butter and baste the fish for 10 seconds. Turn the fillets over, then place the portioned herb crust on top.
Put the fish under the grill until the crust is just crisp. Serve immediately.