Preheat oven to 325 degrees F (165 degrees C). Coat a 13x9 inch baking dish with cooking spray.
In a 3 quart saucepan, boil chicken for about 30 minutes.
Drain, and cut into bite sized pieces.
Meanwhile, cook pasta according to directions.
Drain.
In a large bowl, mix together cream of chicken soup, broccoli, celery, sliced mushrooms, seasoned salt, salt, pepper, and onion. Stir in pasta and chicken pieces.