Put a bay-leaf and a chopped onion in two tablespoonfuls of tarragon vinegar, bring to the boiling point, strain and cool. Cook together two tablespoonfuls of butter and one of flour, add a half cupful of cold water, and cook until thick, stirring constantly. Take from the fire and add the yolks of two eggs beaten with the vinegar. Reheat for a moment, seasoning with salt and pepper, strain, and serve immediately. Lemon-juice may be used in place of the vinegar.