Melt butter in a large stockpot coated with cooking spray over medium-high heat. add onion, celery, pepper, salt, and mushrooms, and saute 4 minutes or until mushrooms are tender. add sherry; cook 1 minute.
Lightly spoon flour into a measuring cup; level with a knife. Gradually add flour to pan; cook 3 minutes, stirring constantly with a whisk. Gradually add broth, stirring constantly. bring to a boil. reduce heat; simmer 5 minutes, stirring frequently. remove from heat.
Add 1 3/4 cups parmesan cheese and cream cheese, stirring constantly until cream cheese melts. add pasta and chicken and stir until blended. Divide pasta mixture between 2 -8 ounce square baking dishes coated with cooking spray.
place bread in food processor, and pulse 10- times or until coarse crumbs form. combine breadcrumbs and remaining 1/2 cup parmesan cheese, and sprinkle evenly over pasta mixture. bake for 30 minutes remove from oven cool slightly and serve