1. In a saucepan, heat the olive oil. Add the onion and cook for 1 minute. Add the bell pepper and zucchini pieces; cook for 1 minute.
2. Add the vegetable broth and bring to a boil. Cover and simmer for 30 minutes.
3. Transfer to a bowl and refrigerate for 8 hours.
4. Remove from the refrigerator. Season with salt and pepper and add the chopped basil.
5. Pour into glasses and add ice cubes. Serve immediately.