Preheat oven to 350.
Melt butter in large stockpot coated with cooking spray over medium-high heat. Add onion, celery, pepper, salt and mushrooms; sautÃ© 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 (mixture will be thick) 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 with a whisk until cream cheese melts. Add the pasta and chicken, and stir until blended. Dived the pasta mixture between 2 (8-inch-square) baking dishes coated with cooking spray.
Place bread in food processor; pulse 10 times or until coarse crumbs form. Combine breadcrumbs and 1/2 cup Parmesan cheese; sprinkle evenly over pasta mixture.
Bake at 350 for 30 minutes or until lightly browned. Remove casserole from oven; let stand 15 minutes. Yield: 2 casseroles, 6 servings each.
To freeze unbaked casserole: Prepare through Step 5. Cool completely in refrigerator. Cover with plastic wrap, pressing to remove as much air as possible. Wrap with heavy-duty foil. Store in freezer for up to 2 months.
To prepare frozen unbaked casserole: Thaw casserole completely in refrigerator (about 24 hours). Preheat oven to 350Âº. Remove foil; reserve foil. Remove plastic wrap; discard wrap. Cover casserole with reserved foil; bake at 350Âº for 30 minutes. Uncover and bake an additional 1 hour or until golden and bubbly. Let stand 15 minutes.
This recipe is also fantastic with leftover turkey.