Preheat oven to 350 degrees F.
Cook pasta to al dente, according to package instructions, then drain and set aside.
Heat a large saucepan over medium heat and add the ground sausage. Cook while stirring until the sausage is no longer pink. Remove cooked sausage, drain on a paper towel lined plate, and set aside.
To the same pan, add the diced onions, salt, pepper, and bell peppers with a tablespoon of olive oil. Cook over medium heat until the onions are translucent and the peppers are soft.
Add the cooked sausage, marinara sauce, and cream cheese and bring the mixture to a boil. Reduce heat slightly, stir to melt, then add the cooked pasta, Parmesan cheese, and half of the mozzarella cheese. Mix to combine.
Spray a 9" x 13" baking dish with nonstick cooking spray and add the pasta mixture, then top with the rest of the mozzarella cheese.
Bake uncovered in a preheated oven for 20-25 minutes, or until the cheese on top is lightly golden and the sauce is bubbling.
Remove from the oven and let cool for at least 5 minutes before serving. Enjoy!