Bring a large pot of salted water to a boil. Cook the penne or rigatoni until just al dente according to the package directions, reserve about 1/2 cup pasta water, and drain.

Heat a large skillet over medium-high heat. Add the sausage and cook for 5–7 minutes, breaking it into small pieces, until browned and cooked through; drain excess fat if needed.

Add the diced onion and cook for 3–4 minutes, until softened. Stir in the garlic, Italian seasoning, and optional red pepper flakes, cooking for about 30 seconds.

Pour in the chicken broth and tomato sauce or crushed tomatoes. Stir while scraping up the browned bits from the skillet, then simmer for 5–7 minutes so the sauce reduces slightly.

Lower the heat and stir in the heavy cream. Simmer gently for 2–3 minutes, keeping the sauce below a rapid boil.

Add the drained pasta and Parmesan to the skillet. Toss until coated, adding reserved pasta water 1–2 tablespoons at a time if the sauce is too thick.

Taste and adjust the salt and black pepper. Finish with chopped parsley or basil and additional Parmesan before serving.