This sauce is a throwback to classic sauces from the 18th century. The sauce is distinctly sour from the mustard and vinegar, but the texture is smooth. It is best served with cold meats./ The Noilly Prat is a brand of French vermouth with its own distinct flavor, but other dry, white vermouths or even dry, white wines can be substituted for it in this recipe./The beurre manié is prepared by combining equal amounts of flour and soft butter into a paste. The paste can be refrigerated for long periods until needed for use. The beurre manié is used to thicken sauces at the last minute. It is whisked into the sauce in small amounts until the desired thickness is achieved. The sauce must come to a simmer before the beurre manié will start to thicken the liquid.
Combine the mustard with a little vinegar. When dissolved, add the mixture to a saucepan with the remaining vinegar, the shallots, Noilly Prat, and chicken stock. Bring to a boil and reduce by about two-thirds.
Just before serving, thicken the sauce by whisking in some beurre manié into the simmering sauce until the desired thickness is achieved. Add the chives and parsley, adjust the seasoning, and serve immediately to maintain the color of the herbs.