Roasting the vegetables gives the soup a delicious flavour. Simply halve the onions, tomatoes and peppers, place them on a lined baking tray. Drizzle with a little olive oil and roast in a 200°C (gas mark 6) oven for 60 minutes, turning the onions and peppers once. Next, peel the skin from the cooked peppers and tomatoes. Add the vegetables to the soup.