In a medium stockpot, heat olive oil over medium heat.
Add onion, and saute until translucent, about 5 minutes.
Add garlic and zucchini, and saute for an additional 2 minutes.
Add broth, bring to a boil.
Reduce to a simmer, cover, and cook for 10 minutes.
Remove from heat, uncover, and allow to cool for 1 hour.
Transfer soup to blender or food processor in small batches; puree.
Repeat with remaining soup.
If serving chilled, place in a bowl, cover, and chill.
If serving warm, return to stockpot, and heat through.
For either recipe, add Greek yogurt before serving, stirring well to combine.
Season with salt and pepper to taste.
Offer hot sauce when serving, if desired.