While the pasta cooks, heat olive oil in a large skillet over medium heat. Add the Italian sausage and cook, breaking it up with a spoon, until browned and cooked through. Remove the sausage from the pan and set it aside, leaving the drippings in the skillet.

Add the chopped onion to the skillet and cook until soft and translucent, about 3 to 4 minutes. Stir in the mushrooms and cook until they release their moisture and begin to brown. Add the garlic and cook for about 30 seconds, just until fragrant.

Pour in the chicken broth and scrape up any browned bits from the bottom of the pan. Lower the heat and stir in the heavy cream, Parmesan cheese, crushed red pepper flakes, salt, pepper, and Italian seasoning. Let the sauce simmer gently until it thickens slightly.

Return the sausage to the pan and stir to combine. Add the cooked rigatoni and toss until everything is coated in sauce. If needed, add a splash of reserved pasta water to loosen the sauce.

Taste and adjust seasoning. Finish with fresh parsley and extra Parmesan before serving.