Boil a large pot of salted water and cook the rigatoni until al dente, about 10-12 minutes. Reserve ½ cup of pasta water, then drain the pasta.

In a skillet, heat olive oil and brown the Italian sausage for 6-8 minutes, breaking it apart as it cooks. Set aside but leave the fats in the pan.

Sauté the diced onion in the same skillet for 3-4 minutes until translucent, then add minced garlic and cook for another 30 seconds.

Add heavy cream and chicken broth to the skillet, stirring to combine, and let simmer for 3-5 minutes until slightly thickened.

Return the browned sausage to the skillet and simmer for another 2-3 minutes to combine flavors.

Mix in grated Parmesan cheese until melted and smooth, and season with salt and black pepper to taste.

Add the drained rigatoni to the sauce, tossing gently to coat. Add reserved pasta water if needed for consistency.

Finish with fresh basil and red pepper flakes, tossing to distribute evenly, then serve immediately.