Bring a large pot of water to a boil and add spaghetti. While the pasta is cooking, slice sausage, green peppers (clear out seeds and white part), and onions. Place in a large saute pan and heat over medium until sausage is browned and onion is soft.

Strain and rinse cooked spaghetti then place in the saute pan. Add marinara sauce and toss together with spaghetti, sausage, and vegetables to heat. Top with Parmesan and serve.