Place the fish
in a frying pan and heat half the butter, the milk,
fish stock, bay leaf and seasoning, and heat over a
moderately low heat to simmering point. Lower the
heat, cover the pan with a tight-fitting lid and
poach the fish for 10-15 minutes, depending on the
thickness of the fillets, until the fish is tender
and the flesh just begins to flake.
Transfer the fish to a warmed serving plate with
slotted spoon, cover the fish and keep warm while
you make the sauce. Return the cooking liquid to the
heat and bring to the boil, stirring. Simmer for
about 4 minutes, then remove and discard the bay
leaf.
Melt the remaining butter in a saucepan and add the
flour, stirring continuously for 1 minute. Remove
from the heat and gradually stir in the fish cooking
liquid. Return to the heat and bring to the boil,
stirring. Simmer for about 4 minutes, stirring
frequently.
Remove the pan from the heat, blend the cream into
the egg yolk, then stir into the sauce with the
parsley. Reheat gently, stirring for a few minutes;
do not allow to boil.
Remove from the heat, add the lemon juice and rind,
and season to taste with salt and pepper. Pour into
a warmed sauceboat and serve with fish.