- Heat oil in a large saucepan over medium heat.
- Add chopped onion and cook, stirring, until beginning to brown, about 4 minutes.
- Add vegetable broth, russet potato(peeled and diced), 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(1/2-inch cubes) 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.