Cook Pasta According To Package Directions. Meanwhile, In A Large Skillet, Saute The Mushrooms And Onion In Butter Until Tender. Add Prosciutto; Saute For 3 Minutes. In A Bowl, Combine The 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 Parmesan Cheese If Desired.