On the Passover Seder plate, horseradish is used as the "maror," the bitter herbs that symbolize the bitterness of slavery. You can serve the horseradish in small chunks on the Seder plate, but it is most commonly ground into a paste. Horseradish is pungent, so take special care when preparing the root. Wear gloves, work in a well-ventilated area and avoid contact with your eyes. A pound of horseradish root will make about 2 cups of ground horseradish.
Cut off any leaves attached to the horseradish root. Clean the root thoroughly with a vegetable scrubber and cold water.
Remove the outer layer of the root with a vegetable peeler.
Cut the horseradish into 1-inch cubes.
Transfer the horseradish cubes to a blender or food processor. Only fill the blender or food processor up halfway, working in batches if necessary. Process the horseradish until it has a smooth, even consistency. Scrape down the sides with a rubber spatula every so often to ensure everything is evenly mixed. You may add a splash of water to help the process along, if you wish.
Add 2 to 3 Tbsp. of white vinegar to the blender for every 1/2 lb. of horseradish root. You may add more or less vinegar, depending on your preference. If you prefer a mild horseradish, add the vinegar immediately after it's finished blending. For a spicier version, wait 3 or 4 minutes before adding the vinegar. Process the horseradish and vinegar for a few seconds until it is thoroughly mixed.
Store the ground horseradish in an airtight container in the refrigerator until you're ready to use it. It will keep safely in the refrigerator for up to six weeks.