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).
Serve warm on any vegetable.
Notes
This is important. Keep whisking sauce over heat and occasionally remove it off the heat to avoid curdling the egg mixture.