Bring a large pot of salted to a boil, cook the rigatoni according to package directions until al dente. Reserve 1-cup of the pasta water before draining. While the pasta is cooking begin to fry the sage leaves and make the sauce.
Line a small plate or baking tray with a few sheets of paper towel. Set aside. Heat the olive oil and butter in a large sauté pan over medium heat. Once the butter is melted and bubbling, gently place the sage leaves into the pan in a single layer. Cook, flipping occasionally until they turn a dark green color, about 1-2 minutes. Transfer the fried sage leaves to the prepared tray. Season with salt.
In the same pan, add the grated garlic to the sage-infused butter and oil. Cook, stirring frequently, until fragrant then pour in the pumpkin purée. Season with a generous pinch of salt and freshly ground black pepper. Cook the pumpkin, stirring frequently, until the color deepens to a dark orange, about 5 minutes. Next, pour in the heavy cream, nutmeg and 1/2 cup of the reserved pasta water, whisk to combine. Bring the sauce to a simmer, then turn the heat down to low, just enough to maintain a slow simmer. Whisk in the grated parmesan cheese then, using a rubber spatular or wooden spoon, gently fold the rigatoni into the sauce together until evenly coated. Add more pasta water as needed to achieve the right consistency.
To serve, divide the pasta between bowls, crumble the crispy sage overtop and sprinkle with extra parmesan cheese and freshly ground black pepper. Serve immediately!