Wash well:

Scrape off the skin. Grate or mince the horseradish (the fine shredding disk of a food processor works well). Bring to a boil in a small saucepan:

Pack the horseradish into a clean pint jar and pour the vinegar mixture over it. Refrigerate overnight. If desired, for horseradish with a creamy consistency, transfer the horseradish to a food processor or blender and process or blend until finely ground, adding a little more vinegar if necessary. Store refrigerated for up to 1 year.