Bring a large pot of lightly salted water to a boil. Cook spaghetti in the boiling water, stirring occasionally, until cooked through but firm to the bite, about 12 minutes. Drain.

Cook and stir sausage in a large skillet over medium-high heat until browned and crumbly, 5 to 7 minutes. Drain and discard grease. Stir onion, bell pepper, and margarine into sausage. Reduce heat to low, cover, and cook until onion is soft, 3 to 5 minutes.

Stir tomatoes, tomato sauce, and Italian seasoning into sausage mixture; bring to a boil. Stir in Parmesan cheese and pour over cooked spaghetti. Top with mozzarella.