Bring a large pot of salted water to a boil. Cook the pasta until al dente according to the package directions. Reserve 1 cup of pasta water, then drain.
Meanwhile, heat a large skillet over medium-high heat. Add the Italian sausage and cook for 5 to 7 minutes, breaking it into small pieces as it browns.
Transfer the cooked sausage to a plate, leaving some of the rendered fat in the skillet.
Add the onion and cook for 3 to 4 minutes, until softened. Add the garlic and cook for about 30 seconds.
Reduce the heat to medium. Add the pumpkin purée, heavy cream, sage, red pepper flakes, black pepper, and salt.
Stir well and gently simmer for 3 to 5 minutes.
Reduce the heat to low and stir in the Parmesan until melted and smooth.
Add the cooked pasta and Italian sausage to the skillet. Toss until everything is evenly coated.
Add reserved pasta water a little at a time until the sauce reaches your preferred consistency.
Taste and adjust the seasoning.
Serve immediately with extra Parmesan, freshly ground black pepper, and fresh sage if desired.