Melt the butter in a large saucepan. In it slowly saute the mushrooms and scallion until tender. Add flour and stir just until flour is cooked, a couple of minutes.
3.
Whisk in first chicken broth and then milk, stirring until thickened. Add salt if desired, pepper, nutmeg if desired, cream cheese and Swiss cheese; stir until melted. Then stir in spinach. Heat and stir very gently.