Heat the oven to 350 degrees. Spray a 3-quart casserole dish with cooking spray.
Cook the pasta as directed on package and drain.
Meanwhile, in a 12-inch nonstick skillet, melt the butter over medium heat. Add the mushrooms and onion. Cook for 3 minutes or until the vegetables starting to soften.
Stir in all the remaining ingredients except the cheddar cheese. Cook until the Italian cheese is melted, stirring constantly.
Pour the mixture over the cooked pasta; mix well. Spoon the mixture into the casserole dish and cover with foil. Bake the casserole for 30 minutes or until thoroughly heated.
Uncover and sprinkle with the cheddar cheese. Return to oven for 5 minutes or until cheese melts.