Coat a large pan with cooking spray and place over medium-high heat. Add butter or margarine. When melted, add mushrooms, onion and celery. Cook, stirring frequently until tender.
Add the flour-milk mixture along with the chicken broth, salt and pepper to the sautéed vegetables. Bring to a boil, reduce heat and simmer for 5 minutes, stirring occasionally.
Spoon the mixture into a shallow baking dish coated with cooking spray. Sprinkle evenly with the remaining Parmesan cheese. Bake for 25-30 minutes or until thoroughly heated.