WHAT YOU DO
Step 1. Over a moderate heat, bring the vinegar, white wine, shallots, tarragon stalks, chervil, if using, and seasoning to the boil and allow to reduce to about 2 tablespoons. Leave to cool slightly.
Step 2. Whisk the egg yolks with 1 tablespoon of water and add to the pan, whisking all the time over a low heat.
Step 3. Once thickened, begin adding the butter in small amounts at a time, whisking as you go. Adjust seasoning. Add the chopped tarragon leaves and keep warm until required, being careful it does not overheat. Strain if desired. Serve with steak, fish, chicken or eggs.
