Melt the butter in a large saucepan. Add the chopped scallions and season with salt and pepper. Cover and cook over low heat, stirring occasionally, until softened, about 10 minutes. Add the flour and cook, stirring, until incorporated. Gradually stir in the stock and bring to a boil, then simmer over low heat for 10 minutes.
Remove the pan from the heat and stir in three-quarters of the mushrooms and all of the parsley. Working in batches, puree the soup in a blender until smooth. Return the soup to the pan and stir in the cream. Season with salt and pepper and warm gently over low heat. Ladle the soup into bowls, garnish with the remaining mushroom slices and the julienned scallion and serve.