onions, parsley, tarragon, chervil, vinegar and water in small saucepan.
Set over low heat and simmer 5 minutes. Strain, saving liquid and
discarding remainder. Put egg yolks in top of double boiler. Add vinegar
mixture to yolks a little at a time, beating constantly with a wire whip
or rotary beater. Set over simmering (it must not be boiling) water and
cook until mixture thickens, stirring constantly. Add butter a little at
a time, stirring until butter is blended after each addition. (Mixture
should look like mayonnaise) Season with salt and cayenne. Serve
secret of success with this sauce is not to let it get too hot (water
should be simmering in bottom of double boiler - not boiling) and to
stir well after each addition of butter. If sauce should separate, a few
drops of cold water should put it back together again. It is usually
made with fresh herbs but we find this method very satisfactory when
fresh herbs are not available.