If you have dried mushrooms soak in heavy cream while you begin preparation on mushrooms. Ideally, these should soak until they are tender. Saute mushrooms and onions over medium heat in a medium sized saucepan. Saute until onions are transparent and the mushrooms have begun to release some of their water. Add 2 cups of chicken stock and potato. Cover and simmer for about 18 minutes over a medium-low heat. Puree soup in either a blender or with a hand immersion blender until soup is well blended and is fairly smooth. Return soup stock to pot, add heavy cream and heat until the soup is hot all of the way through.
You can garnish the soup with a small dollop of sour cream. Chopped fresh chives or fresh dill goes well with this soup.