Preheat oven to 170 degrees C. Coat a rectangular baking dish with cooking spray.
In a 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, salt, pepper and onion. Stir in pasta and chicken. Spread mixture into the prepared baking dish. Sprinkle with grated cheese and paprika.