Boil the pasta until just al dente, according to package directions. Drain and rinse with cool water to stop the cooking, and return to the pan you boiled it in. Set aside.
Preheat the oven to 375 degrees Fahrenheit.
In a medium saucepan over medium heat, add 3 tablespoons olive oil. Add the onion and cook, stirring frequently, until they begin to turn transparent, about 4 minutes.
Add the flour, salt, and pepper. Stir constantly over medium heat for about three minutes.
Stir in the milk and pumpkin. Add the bay leaf.
Keeping on medium heat, whisk constantly for about 10 minutes, until the sauce thickens.
Add the nutmeg and taste. Add additional nutmeg and more salt and pepper if desired.
Remove sauce from heat. Remove and discard the bay leaf.
Pour the sauce over the cooked rigatoni and gently toss until all pasta is coated.
Pour coated pasta into a 5-inch x 8-inch baking dish. Sprinkle with grated Parmesan, and then drizzle with 1 tablespoon olive oil.
Bake for 25-30 minutes until it's bubbly throughout and golden brown on the top.