Combine 1 cup or 2 deciliters of basic white sauce made of fish fumet from poached fish with 1/2 cup or 1 deciliter of fish stock.  Simmer until 1 cup or 2 deciliter remains.  Mix 2 beaten egg yolks with 3 tablespoons (45 grams) of cream. Add to sauce.  Add 3 tablespoons (45 grams) of butter.  This will yield 1 1/2 cups (3 deciliters).