Heat half the oil in a saucepan and sauté the onion, capsicums and zucchini for 5 minutes until softened.
Pour the stock into the pan, add the sugar and bring to the boil. Reduce the heat, then cover and cook gently for 20 minutes. Add the garlic, allow to cool slightly, then tip into a blender or food processor and blend until smooth.
Add the vinegar, paprika and a pinch of salt, then allow to cool. Chill in the refrigerator for at least 2 hours.
Serve the soup in bowls, drizzled with the remaining olive oil and garnished with shredded basil.
When buying capsicums, look for ones with smooth, unblemished skin with a rich colour. Avoid any with wrinkles or soft spots. They will keep in the vegetable crisper for up to a week.