Chicken and Rice Cassarole

In honor of Mr. IM's birthday tomorrow, here is one of his favorite childhood recipes, as made by his mother.  I usually take too many shortcuts and he says mine isn't as good as hers.

This is a good weeknight recipe, as long as some prep is done before.  I usually leave this for a weekend though and make several recipes worth at once to freeze (it freezes well). My shortcuts (that the Mr. objects to) are:  Use 4 chicken breasts, either baked in the oven or (gasp) in the microwave;  use canned mushrooms, use "regular" rice (cook it first, mix it all together)....I've even been known to use celery salt instead of celery.
Enjoy!