Preparation Preheat oven to 350°F. Wrap beets in double thickness of foil. Roast beets on oven rack until tender, about 1 hour 45 minutes. Unwrap beets and cool. Whisk oil, vinegar, salt and pepper in medium bowl until blended. Mix in onion and horseradish. Peel beets; cut into 1/3-inch dice. Add to onion mixture. Cover; chill 1 to 4 days.
Preparation Preheat oven to 350°F. Wrap beets in double thickness of foil. Roast beets on oven rack until tender, about 1 hour 45 minutes. Unwrap beets and cool. Whisk oil, vinegar, salt and pepper in medium bowl until blended. Mix in onion and horseradish. Peel beets; cut into 1/3-inch dice. Add to onion mixture. Cover; chill 1 to 4 days.