Heat the oil in a large pan over a medium heat. Add the onion and fry gently until transparent and softened.
Add the leek and place a lid on the pan. Reduce the heat and cook for 2-3 minutes, or until the leek is softened.
Add the carrots and the stock. Increase the heat to bring to the boil, then reduce the heat to simmer for 20 minutes.
Add the milk and simmer for a further five minutes.
Transfer to a food processor. Blend until smooth.
Allow to cool slightly and add the chopped coriander and season, to taste, with salt and freshly ground black pepper.
To serve, pour the soup into three bowls and serve with a crusty roll alongside.