Heat olive oil in a 4- to 6-quart Dutch oven over medium heat. Add onions and salt to hot oil and cook for 5 minutes or until tender, stirring occasionally. Add beets, potatoes, cranberries, bay leaf, beef broth, and horseradish to Dutch oven. Bring to boiling. Reduce heat and simmer, uncovered, for 30 minutes, stirring occasionally. Add cabbage to soup and cook for 10 minutes more. Remove and discard bay leaf. Top individual servings with sour cream and parsley.