Heat oil over medium-high flame and saute garlic and onion until translucent, about 5 minutes. Add stock and bring to a boil. Reduce heat and simmer about 5 minutes. Let cool to room temperature and puree in blender or food processor for 30 seconds.
Add basil, salt and pepper and blend for another 30 seconds. Add yogurt and pulse to mix. Keep chilled until ready to serve. When serving, add a couple of tablespoons of diced tomato on top of each bowl of soup. Season tomato with salt and serve.
Recipe courtesy of Michael Chiarello