Bring a large pot of salted water to a boil. Once boiling, add the rigatoni pasta and cook according to package instructions until al dente. Drain and set aside.
While the pasta is cooking, heat a large skillet over medium heat. Remove the sausage from its casing and add it to the skillet. Break up the sausage into smaller pieces as it cooks, approximately 5-7 minutes until browned and cooked through.
Add the diced onion to the skillet with the sausage and cook for an additional 3-4 minutes until the onion is translucent.
Stir in the minced garlic and cook for about 1 minute until fragrant.
Pour in the chicken or vegetable broth and bring to a simmer, scraping the bottom of the pan to deglaze any browned bits.
Reduce the heat to low and stir in the heavy cream, grated Parmesan cheese, Italian seasoning, red pepper flakes, salt, and black pepper. Allow the sauce to simmer for about 5 minutes until it thickens slightly.
Add the cooked rigatoni pasta to the skillet, tossing to coat the pasta evenly with the creamy sauce. Cook for an additional 1-2 minutes until everything is heated through.
Remove from heat and serve immediately, garnished with fresh parsley.