Grind the onion and cranberries together. Add all the other ingredients and mix well. Put in a plastic container and freeze. One hour before serving, move to the refrigerator to soften.
NOTES: Frozen cranberry relish with horseradish -- Susan Stamberg, former co-host of National Public Radio's "All Things Considered," used to broadcast this recipe every year around Thanksgiving. She claimed it was her mother's, but it turned out that her mother got it from a Craig Claiborne column many years before. Very few people are neutral about this unusual, rather strong relish, they either love it or hate it. I'm in the former group.
* This relish is shocking pink in color, so plan your table setting accordingly.