1 spring onion, or 1 small onion, finely chopped
2 sprigs fresh chervil
2 teaspoons finely chopped fresh tarragon, or 1 teaspoon dried tarragon
At least 4 crushed black peppercorns, or more according to taste
2 Tablespoons white wine vinegar
2 Tablespoons dry white wine
180g firm butter, cut in pieces
Finely chopped fresh tarragon or parsley, if desired
In a small saucepan, combine spring onion, chervil, tarragon, pepper, vinegar and wine. Cook over moderate heat until liquid is reduced by half.
Place egg yolks in top half of a double boiler over barely simmering water. Pour onion mixture through a fine strainer into egg yolks; whisk until blended. Do not allow water to boil or touch bottom of top half of boiler.
Add butter, piece by piece, whisking constantly. Make sure each piece is melted and absorbed before adding next piece. The sauce should be smooth like mayonnaise. Adjust seasoning with salt and cayenne. Stir in chopped herb, if desired, and serve warm with roast beef, steaks, chicken or fish.
Makes about 500ml (2 cups).