Bring a large pot of salted water to a boil. Add rigatoni pasta and cook according to package instructions until al dente. Reserve ½ cup of pasta water, then drain the pasta and set aside.
In a large skillet, heat olive oil over medium heat. Remove the casing from the Italian sausage and add it to the skillet. Cook the sausage, breaking it apart with a wooden spoon, until browned and cooked through, about 5-7 minutes.
Add the diced onion to the skillet and cook until softened, about 3 minutes. Stir in the minced garlic and Italian seasoning, cooking for an additional 1-2 minutes until fragrant.
Pour in the canned diced tomatoes (with their juices) and bring to a simmer. Reduce the heat to low and stir in the heavy cream, mixing until well combined.
Gradually add the grated Parmesan cheese, stirring until melted and the sauce is creamy. If the sauce is too thick, add some reserved pasta water until the desired consistency is reached. Season with salt and pepper to taste.
Add the drained rigatoni to the skillet, tossing to coat the pasta evenly in the creamy sausage sauce. Cook for 1-2 minutes more to heat through.
Serve hot, garnished with fresh basil or parsley, and additional Parmesan cheese if desired.