Heat the oil in a large saucepan. Tip in the onions, carrot and peppers. Cook gently for 15 mins, to soften. Add the garlic and chilli and cook for a few mins more. Pour over the chopped tomatoes and 800ml of the stock. Bring to the boil and simmer for 10-15 mins until the veg is completely tender.
Meanwhile, mix the yogurt, cucumber and mint in a bowl, and season.
Blitz the soup with a hand blender until smooth, using the extra stock to thin if it has become too thick. Heat through, season and spoon into bowls. Serve with a dollop of the yogurt mixture on top and a drizzle of olive oil.