Remove the stalks from the mushrooms and wash well. Put them in a saucepan with the lemon juice and
Fillet and skin the turbot, cut a strip of about 4 cm/1½ inches wide lengthwise from each of the 2 large fillets. Season these strips, lay them end to end so that they are the length of the terrine and wrap in the saved spinach leaves. (This strip is placed down the middle of the terrine later.)
Cut the remaining turbot into small pieces and purée in a blender or food processor along with the salt until smooth. Add the egg whites and blend for a few seconds more or until well mixed in. Remove from the blender and rub the mixture through a sieve to remove any sinews – only by doing this can you be sure of a really fine texture. Set the bowl on crushed ice and gradually add two-thirds of the cream, followed by the egg yolks, vermouth and seasoning. Test the mousse and, if it is too rubbery, add a little more cream and test again. The texture should be soft, but should still hold together and not break up.
To assemble the terrine, put a