Bring a large pot of water to a boil and season it generously with kosher salt. Add the rigatoni and cook until 1 minute less than al dente. Reserve 1 cup of the starchy pasta water, then drain the pasta and set it aside.
While the pasta cooks, heat 1 to 2 Tablespoons of olive oil in a large skillet over medium heat. Add the mild sweet Italian sausage, breaking it into bite-size crumbles. Cook for 8 to 10 minutes, stirring occasionally, until browned and cooked through. Transfer the sausage to a plate, leaving a little of the fat in the pan.
In the same skillet, add the marinara sauce, heavy cream, Calabrian chili paste or crushed red pepper flakes (if using). Stir and bring to a gentle simmer over medium heat.
Stir in the Parmesan cheese and about 1/4 cup of the reserved pasta water. The sauce should look glossy and pourable. If it looks thick, add more pasta water a splash at a time, up to 1/2 cup total.
Add the drained rigatoni and browned sausage to the skillet with the sauce. Toss gently until everything is evenly coated. If the mixture looks at all dry, add another small splash of pasta water. It should look slightly saucier than you want the finished bake, because the pasta will absorb some of the sauce in the oven.
If you are using a separate baking dish, lightly oil a 9 x 13 inch baking dish. If your skillet is oven-safe and you are baking directly in the skillet, you can skip this step. Spoon the saucy rigatoni and sausage into the baking dish, if using. Sprinkle the mozzarella evenly over the top and drizzle very lightly with olive oil.
Loosely cover the dish with foil. Bake at 400°F (200°C) for about 15 minutes.
Remove the foil and bake for another 5 to 8 minutes, or until the pasta is bubbling around the edges and the top is melted and lightly golden.
Remove from the oven and let the pasta rest for a few minutes. Top with fresh basil, if desired. Serve immediately while hot, for the best texture and flavor.