In a large pot of boiling, salted water, cook the pasta until al dente. Drain, reserving 1 cup pasta cooking water.
In the same pot, melt the butter over medium-low heat. Add the onion and season with salt and pepper; cook, stirring, until softened, about 6 minutes. Stir in the pumpkin and heavy cream and bring to a boil.
Return the pasta to the pot along with the reserved pasta cooking water and toss. Stir in the parmesan; season with salt and pepper.