Cook pasta according to package directions until just al dente. Drain and set aside.
In a saucepan, melt butter then saute mushrooms and onion until both are nice and soft.
In a large bowl mix soup and sour cream until blended. And cooked mushrooms, onion, chicken and spaghetti and fold until everything is evenly coated with sauce.
Pour spaghetti into a 9x13 dish and top with cheese. Bake in a preheated oven for about 30 minutes or until casserole is heated through.