3 tablespoons cold unsalted butter, cut into tablespoons
1/2 cup chervil leaves or 1/4 cup flat-leaf parsley leaves plus 2 teaspoons
In a small saucepan, combine the wine with the shallots, lemon juice and salt and boil over moderately high heat until reduced by half. Turn the heat to low and whisk in the butter, 1 tablespoon at a time, until smooth.
Just before serving, put the chervil in a blender. Reheat the sauce gently over low heat; do not let it boil. Pour the sauce over the chervil, blend until smooth and serve at once.