1. Heat oil in a large saucepan over medium heat. Add onion; sauté 3 minutes. Add broth and next 3 ingredients. Cover and bring to a boil over high heat; reduce heat, and simmer 8 minutes or until potato is very tender, stirring occasionally. Add spinach; cover and cook 2 minutes or until spinach wilts.
2. Place half of soup and 1/4 cup sour cream in a blender. Remove center piece of blender lid (to allow steam to escape); secure blender lid on blender. Place a clean towel over opening in blender lid (to avoid splatters). Blend until smooth. Pour into a large bowl. Repeat procedure with remaining soup and 1/4 cup sour cream.
3. Pour soup evenly into 4 bowls. Top each with a dollop of additional sour cream, if desired, and 1/4 teaspoon lemon rind.