Serve this pretty summer soup in clear glasses.
- Portion size 10 servings
In large heavy saucepan, melt butter and cook onions until tender and translucent, about 8 minutes. Add tomatoes and zucchini; cook, without browning, for 5 minutes, stirring frequently. Add chopped basil and stock; cover and bring to boil. Reduce heat and simmer, stirring periodically, for 30 minutes.
Puree soup in blender, food mill or food processor. Transfer to bowl and chill.
Before serving, stir in yogurt, and salt and pepper to taste. Ladle into chilled glasses and garnish with basil leaves.