Cook spaghetti according to package directions. Meanwhile, in a small skillet, saute onion and green pepper in butter until crisp-tender. Crumble sausage over vegetables; add mushrooms. Cook over medium heat until meat is no longer pink; drain.
Stir in the olives, Italian seasoning and salt if desired. Add cream; bring to a boil. Reduce heat; simmer, uncovered, for 10-12 minutes or until thickened. Drain spaghetti; add sauce and toss gently. Garnish with Parmesan cheese if desired.