Leave the root of the horseradish soaking in cold water for 1 hour, then wash well and scrape clean. Grate the horseradish or cut into very thin shreds with a sharp knife. Whip the cream to soft peaks and fold in the horseradish, sugar, mustard, salt, pepper and vinegar. Serve cold with beef.
This recipe comes from Mary Norwak and the fresh sauce made like this is streets ahead of the shop-bought variety. It can also be made using horseradish pickle in place of fresh horseradish. Rinse it thoroughly before use, squeeze out the excess liquid, and fold it into the cream in the same way.