Combine first 12 ingredients in a Dutch oven; bring to a boil. Cover, reduce heat, and simmer 30 minutes or until vegetables are tender. Transfer mixture in batches to container of an electric blender. Cover and process until smooth. Return puree to Dutch oven. Cook over medium heat just until thoroughly heated (do not boil).
Ladle soup into individual bowls. Top each serving with 2 teaspoons sour cream and 2 tablespoons diced tomato.