Heat the oil in a large saucepan. Add the onion and cook for about 5 minutes, stirring occasionally, or until softened but not browned.
Add the leek, carrot, fennel, swede and potato, then cook for a further 5 minutes or until slightly softened. Tie the bay leaf, thyme and parsley sprigs together into a bouquet garni. Add to the pan, together with the stock and tomatoes. Season to taste and bring to the boil, then cover the pan and reduce the heat. Simmer gently for 45 minutes or until all the vegetables are tender.
Remove the bouquet garni and check the seasoning. Sprinkle the soup with snipped fennel leaves and serve piping hot.