Scale the fish, cut off the fins and spines, then wash and gut it. Keep the liver.
Heat the olive oil in a heavy-based saucepan. Soften the chopped onion for two minutes without letting it brown, then add the diced mushrooms, chervil and tarragon and cook on for two minutes more. Add the finely-chopped liver of the rascasse. Stir it in with the onion mix