Cook pasta according to package directions. Meanwhile, in a large skillet, saute mushrooms and onion in butter until tender. Add prosciutto; saute for 3 minutes.
In a small bowl, combine cornstarch and cream until smooth; stir into the skillet. Add parsley and pepper. Bring to a boil; cook and stir for 2 minutes or until thickened. Drain pasta; add to the skillet and stir to coat. Sprinkle with cheese if desired.