Pre-heat oven to 180 °C / Gas 4.
Put the trout in the middle of a large piece of foil and season with 2 tablespoons of lemon juice and dot with half of the butter.
Seal the foil and place the package on a baking sheet. Cooking time should be calculated at 10 minutes per 450g.
Place trout in the oven and bake the trout until tender.
Take the trout from the foil and save the juices then whilst the fish is still warm remove the skin.
Place on a serving dish and leave to cool.
To Make The Sauce:
Gently heat the juices and the remaining butter in a saucepan. Add the watercress, spinach, parsley, chervil and dill, then cook until softened for around 2-3 minutes.
Put the sauce in a food processor and blend it into a smooth mixture.
Put this in a bowl and add the rest of the lemon juice. Leave to cool, then add the mayonnaise. Refrigerate until required.
Serve and garnish with fresh herbs and whole prawns. Add herb sauce.
