Bring a large pot of salted water to a boil.
In a large skillet over medium-high heat, heat the olive oil. Add the sausage and cook, stirring occasionally, for 5 to 6 minutes, until browned and cooked through. Remove from the skillet and set aside, leaving any drippings in the pan.
Add the bell peppers and onion to the skillet and sauté, stirring occasionally, for 3 to 4 minutes, until slightly tender.
Add the garlic and cook, stirring constantly, for 1 minute more, until fragrant.
Reduce the heat to medium-low and pour in the marinara sauce, stirring to combine. Return the sausage to the skillet and stir to coat. Simmer, stirring occasionally, for 5 to 7 minutes, until the sauce has slightly thickened and the flavors have melded.
Meanwhile, cook the spaghetti according to package directions until al dente.
To serve, either transfer the spaghetti to a large serving dish and ladle the sauce generously over the top, or divide the spaghetti among individual bowls and top each with a generous ladle of sauce. Finish with grated Parmesan cheese and serve immediately.