Some good fish stock made by poaching fish in a good Court Bouillon, preferably with white wine in it (see beginning of Fish section).
This is a very easy sauce to make provided you do not let it boil after adding the eggs.
Boil 2 glasses of fish stock from Court Bouillon on a high heat so that it is reduced by half, strain into a double saucepan, leave it to cool a little, then stir in 2 yolks of egg previously beaten up with a little lemon juice.
Add some capers, and leave till required.
The yolks of egg may be replaced by 2 tablespoonfuls of melted butter mixed with flour.