Heat the oil in a large saucepan and fry the onions gently for 5 minutes, or until softened and golden brown. Add the garlic and cook gently for 1 minute. Add the peppers and half the courgettes to the pan. Fry for 5–8 minutes, or until softened and brown.
Add the stock to the pan, season to taste with salt and pepper and bring to the boil. Reduce the heat, cover the pan and simmer gently for 20 minutes.
Allow the soup to cool slightly once the vegetables are tender, then purée in batches in a blender or food processor. Gently fry the remaining chopped courgette for 5 minutes. Meanwhile, return the soup to the pan, reheat, taste and adjust the seasoning if needed. Serve topped with the fried courgette, yogurt or crème fraîche and chives.