Place 1 cup cucumber, spinach, 2 cups buttermilk, 1/2 cup bell pepper, and the next 5 ingredients (bell pepper through black pepper) in a blender, and process until smooth.
Pour pureed mixture into large bowl; stir in 1 cup cucumber, 2 cups buttermilk, 1/2 cup bell pepper, and onions. Ladle soup into bowls; top with sour cream. Garnish with cilantro leaves, if desired.
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