Cook pasta according to package directions and drain.
Saute sausage in large skillet until cooked. Drain off fat. Add onion and cook until clear. Add sauce and all other ingredients except mushrooms and cheese. Cook until heated thoroughly. In an oversized casserole dish put pasta and sauce mixture and raw mushrooms and Parmesan and mix thoroughly. Slice the Mozzarella and layer it on top. Bake for about 20-30 minutes until sauce is bubbling and cheese melted. If you make it ahead of time and refrigerate it, it has to bake longer and then cover it loosely with foil so the Mozzarella doesn't harden. OR you can leave Mozzarella off until last 15-20 minutes or so of baking.