Combine the dried mushrooms and boiling water in a small bowl and stand for 10 to 20 minutes or until softened. Reserving the liquid, scoop out the dried mushrooms and chop coarsely. Strain the soaking liquid through a coffee filter or a paper towel-lined sieve.
Heat the oil in a large saucepan over moderate heat. Add the onion and garlic and cook for 5 minutes or until the onion has softened. Add the shiitakes and cook for 5 minutes. Add the button mushrooms and cook, stirring frequently, for 5 minutes. Stir in the rehydrated porcini mushrooms.
Add the reserved soaking liquid and bring to the boil. Add the sherry and cook for 5 minutes or until the liquid has reduced by half. Add the chicken stock, tomato, tarragon, pepper and 1½ cups water. Bring to the boil, reduce to a simmer, cover and cook for 10 minutes or until the soup is richly flavoured.
Very low in kilojoules, yet packed with meaty flavour, mushrooms also contain substances that may enhance the immune system and slow tumour growth. Cooking mushrooms breaks down their fibrous cell walls, making some of their nutrients more available to the body.