1Bring about 2 cup of water to boil. Place dried mushroom in a large glass bowl or measuring cup.
Pour the boiling water over the mushrooms and set aside.
Tap or gently rinse the dirt off the fresh mushrooms, cut into small pieces, and set aside.
2Heat oil and butter in large skillet over medium heat. Add onions and cook, stirring occasionally, until onions are translucent about 3 minutes.
Add the celery, fresh mushrooms, and rosemary and continue to cook a couple of more minutes.
Transfer the mixture to the slow cooker.
Add the sherry and porcinis with the broth.
Cover and cook on Low for 3-4 hours or on High for about 2 hours.
3Using a standard or immersion blender , puree soup until smooth.
If using a standard blender, puree in small batches to prevent spillage. Add tarragon and milk just before serving, and season with slat and pepper.