Bring a large pot of salted water to a boil.
Cook the rigatoni according to package directions until al dente.
Reserve 1/4 cup of pasta water and drain the pasta.
Meanwhile, melt the butter in a large skillet over medium heat.
Cook the butter for 3 to 5 minutes, stirring frequently, until golden brown and nutty.
Add the garlic and cook for 30 seconds.
Stir in the pumpkin puree, heavy cream, salt, pepper, nutmeg, sage, and red pepper flakes.
Cook for 3 to 4 minutes until smooth and heated through.
Add the Parmesan cheese and stir until melted.
Mix in the cooked pasta and toss until coated.
Add reserved pasta water as needed to loosen the sauce.
Garnish with fried sage leaves and additional Parmesan cheese.
Serve immediately.