Chop onion, and sauté in 1 tablespoon olive oil until golden. Add beets, broth, and 1/2 teaspoon salt. Bring to a boil. Reduce heat, and simmer 20 minutes, or until beets are tender. Purée mixture in a blender until smooth. Whisk 1/2 cup hot soup into 1 cup yogurt, and stir yogurt mixture into soup, until well-blended.