01 - Boil pasta in a large pot of salted water until al dente according to package directions. Reserve 1/2 cup cooking water, then drain and set pasta aside.
02 - Heat olive oil and butter in a large skillet over medium heat. Add chopped onion and cook for 3 to 4 minutes until translucent.
03 - Stir in minced garlic and chopped sage; cook for 1 to 2 minutes until aromatic.
04 - Add pumpkin purée, salt, black pepper, and nutmeg. Stir frequently and cook for 2 to 3 minutes.
05 - Pour in heavy cream and stir until the mixture is smooth. Simmer gently for 5 minutes to develop flavors.
06 - Stir in grated Parmesan cheese until melted. If sauce is too thick, add reserved pasta water a little at a time until desired consistency is reached.
07 - Add drained pasta to the skillet and toss to coat evenly with sauce. Warm through for 1 to 2 minutes.
08 - Plate immediately, garnished with additional Parmesan and sage if preferred.