Turban of Sole with Mousse

Bring the fish stock, water, butter and one teaspoon salt to a boil in a saucepan. As soon as butter has melted, remove the mixture from the heat and add the flour all at once. Beat vigorously with a wooden spoon to blend. Return saucepan to heat and continue beating over moderate heat until mixture leaves the sides of the saucepan and begins to film the bottom. Remove f