Boil one pint of Bechamel Sauce with one gill of the fumet of that fish. Reduce by a good quarter, and add two ounces of Gruyere and two ounces of grated Parmesan.

Put the sauce on the fire again for a few minutes, and melt the cheese by stirring with a small whisk. Finish the sauce away from the fire with two ounces of butter added slowly.