Crapaudine Sauce

A five-minute finishing sauce that assumes you already have a piquante sauce on hand, which is where nearly all the work sits. Read it as an embellishment of a mother sauce rather than a standalone recipe.

• Half a pintOne US cup, 240 ml. "Light" here most likely means thin in body rather than pale in colour; a piquante that has been reduced to a coating consistency can be loosened with a little stock before the mushrooms go in.
• Piquant sauceIn classical French practice, sauce piquante is shallots sweated and deglazed with vinegar and white wine, reduced, then bound with demi-glace and finished with chopped gherkins and herbs. A modern cook can build it on a good reduced brown stock; a jarred brown gravy will taste of nothing but salt under the vinegar.
• The mushroomsFour chopped mushrooms is roughly 2 to 3 oz of button or cremini. Chop them fine and expect them to release liquid.
• Mustard, and when to add itDiluting the mustard in tarragon vinegar is the sound part of the instruction. Mustard held at a boil loses its sharpness quickly, so if you want the bite to carry, stir it in during the last minute rather than at the start of the five.
• Five minutes of boilingThat is long enough to cook the raw edge off the mushrooms and to sharpen the sauce by reduction. Keep it at a lively simmer rather than a rolling boil if the base is already well reduced, or it will turn gluey.
• YieldAbout a cup, enough for four small birds or two larger ones. It is a spooned-over or passed sauce, not a napping sauce.
• What to serve it withCrapaudine is the classic accompaniment to poultry split, flattened and grilled -- squab, poussin, or a spatchcocked chicken. The vinegar and mustard are there to cut charred skin, so it does little for a poached or steamed bird.