Bring a large pot of salted water to a boil. Cook the rigatoni until just shy of al dente, following the package timing. Reserve 1 cup pasta water, then drain.

While the pasta cooks, heat the olive oil in a large skillet over medium-high heat. Add the sausage and break it into small, chunky crumbles.

Cook for 6 to 8 minutes, stirring occasionally, until the sausage is deeply browned and reaches 160°F. Transfer it to a plate, leaving the browned bits in the skillet.

Reduce the heat to medium. Add the onion and cook for 3 minutes, until softened. Stir in the garlic, Italian seasoning, red pepper flakes, remaining salt, and pepper; cook for 30 seconds.

Pour in the chicken broth and scrape up the browned bits. Simmer for 2 minutes, until slightly reduced.

Add the heavy cream and bring it to a gentle simmer. Cook for 3 to 4 minutes, stirring often, until the sauce lightly coats a spoon.

Turn the heat to low. Stir in the Parmesan a little at a time until smooth. Don't let the sauce boil after adding the cheese.

Add the rigatoni and sausage to the skillet. Toss until coated, adding reserved pasta water a splash at a time if the sauce is too thick.

Rest the pasta for 2 minutes off the heat, then scatter with parsley and serve hot.