Rosseberry Sauce

A cold mustard-horseradish sauce of the roast beef and boiled brisket school, and one of the shortest entries in Ranhofer's long catalogue of cold sauces. Nothing is cooked, so the whole thing rests on the mustard being hydrated properly before anything else goes in.

• GillA gill is half a cup, 4 fluid ounces. That is a great deal of vinegar for one tablespoon of mustard, and the finished sauce is thin and pourable rather than a thick paste.
• English mustardThis means dry mustard powder, not the prepared yellow jar; Colman's is the same style of mustard flour and was already on sale in Ranhofer's day. Stir it into the vinegar first and let it stand a few minutes so the powder hydrates and loses its raw, dusty taste.
• CoffeespoonfulBy the usual convention a coffeespoon is smaller than a teaspoon, roughly half a teaspoon. The sugar is there to take the edge off the vinegar, not to sweeten; start low and taste.
• Fresh horseradishGrate it on a microplane just before mixing and keep your face out of the fumes. Bottled prepared horseradish is already packed in vinegar and much milder, so if you substitute it, use half again as much and cut the vinegar back.
• VinegarThe recipe specifies only "good vinegar" and names no type. White wine vinegar suits it; distilled white vinegar will work but is harsher, and cider vinegar shifts the sauce sweeter.
• TimingBoth mustard and grated horseradish lose their pungency to acid and air within a few hours. Mix it the same day it is served, and keep it covered and cold in between.
• Yield and servingAbout two-thirds of a cup, enough for six as a condiment. It is intended for cold roast beef, boiled beef, smoked tongue or ham, spooned over or served alongside rather than folded in.