Method

Place finely chopped onion or shallot in a small pan.

Add vinegar and white wine with dried or fresh tarragon

Bring to boil and reduce slowly to by just over a half. It is best to end up with about 25ml.

Strain out the onion and tarragon, saving the reduction.

Melt the butter, slowly, without stirring, in a small pan with a spout. It is important to melt slowly so that the butter oil and sediment are not stirred together. Keep hot but do not allow to boil.

Place the egg yolk in a small bowl, rest the bowl over a smaller one with some boiling water in it. (I use bowl which is for the finished sauce)

Add 15ml of reduced vinegar.

Whisk well together and then add the butter oil very slowly, whisking constantly. This is just like making mayonnaise.

The hot butter along with the heat from the hot water will help to emulsify the sauce.

Continue to add the butter oil slowly until the required consistency is reached. If the sauce is becoming too thick add a little more vinegar reduction.

Once all the butter or sauce is thick enough add enough chopped fresh tarragon to taste.

Season the sauce and serve immediately.