2 tablespoons extra-virgin olive oil
1 medium onion, chopped
2 cups reduced-sodium beef broth, or vegetable broth
1 medium russet potato, peeled and diced
1/2 teaspoon salt
1/4 teaspoon freshly ground pepper
1 1/2 cups steamed cubed beets, 1/2-inch cubes
2 teaspoons red-wine vinegar
1/4 cup reduced-fat sour cream
1 tablespoon prepared horseradish
1 tablespoon fresh chopped parsley
Heat a large saucepan over medium heat. Add onion and cook, stirring, until beginning to brown, about 4 minutes. Add broth, potato, salt and pepper; bring to a boil. Reduce heat to a simmer, cover and cook until the potato is just tender, about 8 minutes. Add beets and vinegar; return to a boil. Cover and continue cooking until the broth is deep red and the potato is very soft, 2 to 3 minutes more.
Combine sour cream and horseradish in a small bowl. Serve the soup with a dollop of the horseradish sour cream and a sprinkle of parsley.