In a large skillet cook the onion, the bell pepper, and the garlic in the butter over moderate heat, stirring, until the vegetables are softened. Stir in the pumpkin, the broth, the water, the cream, the nutmeg, and salt and pepper to taste, and simmer the sauce, stirring occasionally, for 10 minutes.
While the sauce is simmering, in a kettle of salted boiling water boil the penne until it is al dente. Reserve about 1 cup of the cooking water, and drain the penne well.
Add the penne to the sauce, cook the mixture over moderate heat, stirring and thinning the sauce as desired with some of the reserved cooking water, for 1 to 2 minutes, or until the pasta is coated well, and stir in the parsley. Divide the pasta between 2 plates and serve it with the Parmesan.