In a glass, dissolve 1 1/2 teaspoons of corn starch ( or flour) in 2 teaspoons of water to make a runny paste.
In a quart bowl, thoroughly beat the egg and lemon juice and whisk in the corn starch.
Pour egg and lemon mixture into a small saucepan and slowly add half the warm chicken stock. Whisk thoroughly while doing so.
Put the saucepan on a low heat and keep srirring the sauce. When it starts to thicken (about a minute), add remaining stock in small batches and stirring thoroughly while doing so. Remove from heat when the sauce is as thick as heavy whipping cream. ( CAUTION. DO NOT LET THE SAUCE COME TO A BOIL AND BE PREPARED TO REMOVE IT FROM THE HEAT IF IT THICKENS TOO QUICKLY - stir in a little stock or water if it does).