In a large skillet, saute onion and green pepper in butter until tender. Add the tomatoes, mushrooms, olives, oregano and, if desired, ground beef. Simmer, uncovered, for 10 minutes. , Place half of the spaghetti in a greased 13x9-in. baking dish. Layer with half of the vegetable mixture and 1 cup cheddar cheese. Repeat layers. , In a small bowl, combine soup and water until smooth; pour over casserole. Sprinkle with Parmesan cheese. Bake, uncovered, at 350° until heated through, 30-35 minutes.