Heat the oil in a Dutch oven over medium-high heat. Add onion; sauté 3 minutes or until tender. Add broth and next 6 ingredients (through bay leaf). Bring to a boil; reduce heat, and simmer, uncovered, 20 minutes or until beets and potato are tender. Discard bay leaf.
Place one-third broth mixture in blender or food processor; process until smooth. Place puréed mixture in a large bowl. Repeat procedure twice with remaining broth mixture. Return pureed mixture to pan. Warm soup over low heat for 5 minutes or until thoroughly heated. Remove from heat, and stir in lemon juice.
Combine 1/2 cup soup and the sour cream, stirring with a whisk. Divide soup evenly among each of 8 bowls. Top each serving with 1 tablespoon sour cream mixture; swirl sour cream mixture using the tip of a knife.
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