Following the technique for sauce parisienne, cook the flour and butter slowly together until they foam and froth for 2 minutes. Off heat, beat in the hot fish cooking liquid, then the milk. Boil 1 minute. Beat the hot sauce by droplets into the yolks and cream. Return mixture to saucepan and boil, stirring, for 1 minute. Thin out with more cream if necessary, and correct seasoning. Strain. Film top of sauce with a tablespoon of melted butter if not to be served immediately.