Bring a large pot of water to a boil. Add 8 ounces pasta and cook according to package directions. Before draining, reserve 1/2 cup of the pasta cooking liquid, then drain the pasta.
If the 1 pound hot Italian sausage is in casings, remove them. Heat a large skillet over medium-high heat and add a drizzle of olive oil. Add the sausage, breaking it up with a spoon, and cook until browned and cooked through, about 6–8 minutes. Transfer the cooked sausage to a plate and set aside.
In the same skillet, add a drizzle of olive oil if the pan is dry. Add the thinly sliced 1/2 medium onion and the thinly sliced 1 green bell pepper. Cook over medium heat, stirring occasionally, until softened, about 5–7 minutes. Add the minced 3 garlic cloves and cook until fragrant, about 30 seconds.
Add the drained 1 (14.5-ounce) candiced tomatoes, 2/3 cup heavy cream, 1 1/2 teaspoons cajun seasoning, 1/4 teaspoon cayenne pepper, and 1/2 teaspoon black pepper to the skillet. Stir to combine and bring to a gentle simmer. Let simmer a few minutes until the sauce thickens slightly, about 2–3 minutes.
Return the cooked sausage and the drained pasta to the skillet. Stir to coat everything in the sauce. If the pasta seems dry, add some of the reserved 1/2 cup pasta liquid a little at a time until you reach the desired consistency.
Stir in 1/3 cup freshly grated Parmesan cheese and 1 tablespoon chopped fresh parsley until the cheese melts and the sauce is creamy.
Remove from heat and serve immediately.