In a small bowl, whisk together the sour cream, cilantro, and garlic to blend. Season with salt and pepper. (can be made 1 day ahead; cover and chill)
PREPARE THE SHRIMP:
In a medium bowl, combine the shrimp and the marinade ingredients; cover and chill for at least 4 hours- or overnight (can be made 1 day ahead; keep chilled)
PREPARE THE SOUP:
Heat the oil in large, heavy pot over medium heat. Add the onion; sauté until tender, about 8 minutes. Add the garlic; stir 1 minute. Add the zucchini; stir to coat. Stir in the broth; bring to a boil. Cover, reduce heat to medium-low, and simmer until zucchini is tender, about 10 minutes. Cool to lukewarm. Add the chopped cilantro. Working in batches, puree the soup in a blender until smooth.
Transfer the soup to a large bowl. Season to taste with salt and pepper. Cover and chill until cold, about 4 hours. (can be made 1 day ahead; keep chilled)
Divide the soup among bowls, 6 if you're serving as an appetizer and 4 for a main dish. Top each with shrimp, dividing equally. Spoon dollop of cilantro cream over each, garnishing with cilantro sprigs. Serve immediately.
Notes
If preparing this recipe as GLUTEN FREE, just be sure to use a brand of chicken broth that is designated as GF.