Combine the wine vinegar, 2tbsp snipped tarragon, the shallot and peppercorns in a small, heavy-based saucepan and reduce by half over a low heat. Set aside to cool.
When the vinegar reduction is cold, add the egg yolks and 3tbsp cold water. Set the pan over a low heat and whisk continuously, making sure that the whisk reaches right down into the bottom of the pan. As you whisk, gently increase the heat; the sauce should emulsify slowly and gradually, becoming oily after 8-10 mins. (Do not let it become hotter than 65ºC.)
Turn off the heat and whisk the clarified butter into the sauce, a little at a time.
Season with salt and pepper and pass the sauce through a conical sieve into another pan. Stir in the rest of the tarragon, the chervil and lemon juice. Check the seasoning and serve at once.
This recipe is taken from Michel Roux Sauces, Quadrille Publishing Ltd, RRP £14.99, available from Amazon