Take the fish out of the packaging. Remove the head with a sharp knife and carefully cut along the line of the central bone to separate the meat from the bones.
Separate the egg yolk from the egg white and put the yolk into a dish. Add a pinch of salt, pepper and the mustard. Use a whisk for stirring. Mix well and pour the oil in little by little, in a thin stream, stirring constantly. You will need 100–150 ml for a very thick mayonnaise. Add the chopped capers, parsley, sour cream and several drops of lemon juice. Mix well. Add some more salt or pepper if needed. Keep it in the fridge until ready to serve.
Take the fish out of the packaging. Remove the head with a sharp knife and carefully cut along the line of the central bone to separate the meat from the bones. Follow the line of the central bone with the knife to separate the first fillet. Repeat the procedure on the other side of the fish. Keep the head and the central bone to use later for fish stock. Use fish tweezers to remove all remaining bones from the fillet.
Season the fish with salt, roll it into the egg and then into the polenta to which the herbs have previously been added. Fry on well-heated olive oil on medium heat, 2–3 minutes on each side. Serve with a salad and the sauce.