Mustard Sauce

Sauce Moutarde teaches the quiet discipline of finishing an emulsion away from heat. The mustard isn't cooked into submission. It is whisked into warm butter sauce so its bite stays clear while the sauce remains glossy. One true thing before you touch the pan: once the mustard goes in, boiling is forbidden.

The original formula assumed a saucier with finished butter sauce ready to hand, enough service to use any necessary quantity, and a bain-marie waiting beside the range. At home, use about two quarts of prepared butter sauce, keep the book's exact ratio of one tablespoon of mustard per pint, and turn a wide skillet of warm water into the bain-marie, a hot-water bath for holding the sauce. The service scaffolding goes; the off-fire finish and the ratio stay, because they are the dish. One cook, one stove, one evening.

Expect an ivory sauce with a Dijon-gold cast, rich first and brisk at the finish, made for grilled small fish and fresh herring above all. The one step that matters is the last one: lift the pan, whisk in the mustard off the fire, and never let the Sauce Moutarde boil while it waits.