Heat the oil in a large saucepan. Add the leeks and cook gently for 10 minutes until softened.
Tip the chopped tomatoes into a blender or food processor and whizz them into a pulp.
Finely chop the parsley leaves. Tie together the parsley stalks, celery, thyme and bay leaf to make a bouquet garni.
Add the tomatoes to the leeks, together with the tomato puree, chopped parsley, garlic, sugar and bouquet garni. Season with salt. Cover and simmer gently for 15 minutes. Remove the bouquet garni.
Add the vermicelli or spaghetti to the soup and cook for 5 minutes until just tender. Add the butter, stir and season with pepper. Ladle into soup bowls and serve garnished with thyme.