Horseradish Vinegar is vinegar flavoured with horseradish and a few additional seasonings.
You started with freshly grated horseradish from a horseradish root. For this reason, most recipes advise that it be made in the late fall, when horseradish roots are at their peak.
Some people, like Mrs Beeton, called for the addition of minced shallot and paprika. Others substituted ground black pepper for the minced shallot. Some suggested that instead of horseradish, you could use minced shallot for a milder flavour.
You warm the vinegar, and put all the ingredients in a bottle. Directions vary, but as an example, you let it steep for anywhere from 7 to 14 days, shaking it every day, then strain it into a clean bottle to store and use.
You can use it on cold beef, or add to gravy.
Literature & Lore
“448. INGREDIENTS. –1/4 lb. of scraped horseradish, 1 oz. of minced shalot, 1 drachm of cayenne, 1 quart of vinegar.
Mode. Put all the ingredients into a bottle, which shake well every day for a fortnight. When it is thoroughly steeped, strain and bottle, and it will be fit for use immediately. This will be found an agreeable relish to cold beef, &c.”
— Mrs Isabella Beeton. Book of Household Management. October, 1861. Recipe 448.