Source: The Rodale Whole Foods Cookbook
Wash and peel horseradish. Using the finest grater on your food processor grate the horseradish. While the machine is still running add the vinegar until you have a thick paste. Place in a container and cover. This paste will keep almost indefinitely in the refrigerator. Use prepared horseradish to accompany roast beef or in mashed potatoes. Mix it with mayonnaise to spread on sandwiches or used in potato salad.