How to Make It
Position knife blade in food processor bowl; add cranberries and onion. Process 10 seconds, stopping once to scrape down sides of bowl. Pulse 3 or 4 times or until finely chopped, but not smooth.
Combine cranberry mixture, sugar, sour cream, and horseradish in a medium bowl. Spoon into an 8-inch square pan. Cover and freeze until firm.
Remove from freezer, and break into chunks about 1 hour before serving. Store any remaining relish in freezer.