Put shallots, 1/2 the tarragon, and chervil, the thyme, bay leaf, vinegar, wine and a pinch of salt in a small saucepan and boil until reduced to 3/4 tablespoon.
Remove from heat, strain and leave to cool for a minute.
Add the reduction, 1/2 tablespoon of cold water and the egg yolk to the top
of a double boiler. Whisk constantly over hot but not boiling water. Add the
butter a few pieces at a time and whisk until smooth before adding more butter.
When the sauce is thick, add the rest of the tarragon, chervil, lemon juice
and salt & pepper to taste.