1Drain beets, reserving juice. Combine the liquid with sugar, mustard, vinegar and celery seeds in a saucepan over medium heat. Stir to dissolve sugar and mustard. Add onion, salt and pepper; simmer 3-4 minutes, or until onion is slightly wilted. Remove from heat.
2Combine with beets in a mixing bowl. Cool. Cover and refrigerate for at least 3 hours.