In a small bowl, combine dried mushrooms and 1 cup boiling water and let stand 10 minutes or until softened. Using a slotted spoon, remove mushrooms, then coarsely chop. Strain soaking liquid through a coffee filter or a sieve lined with paper towels.
Heat oil in a large saucepan over medium heat. Add onion and garlic and sauté 5 minutes or until onion is softened. Stir in dried mushrooms, add shiitakes, and sauté 5 minutes or until softened. Add button mushrooms and sauté 5 minutes or until they begin to release their liquid.
Add soaking liquid and bring to boil. Add sherry and cook 5 minutes or until reduced by half. Add 1 1/2 cups water, stock, tomato, tarragon and pepper. Bring mushroom soup to boil, then reduce heat and simmer, covered, 10 minutes.