Cook rice according to package directions.
Brown sausage in skillet with onion, drain grease.
In a large mixing bowl, add sausage, cooked rice, soup and sour cream. Mix together thoroughly.
Add mixture to a 9x13 baking pan and spread even.
Add sliced mushrooms and shredded cheese on top.
Bake for 30 minutes or until cheese is melted and mixture is bubbling.
