Boil the water. Set a large pot of salted water over high. Bring to a rolling boil before adding the pasta.
Note: We recommend starting the sauce while the water comes to a boil so the sauce and pasta finish at the same time, check the notes below for more tips on timing.
Cook the pasta. Stir the pasta occasionally after adding to prevent sticking together. Cook the pasta for 1 to 4 minutes, the pasta will start to float at the surface of the water and the texture will be tender but not mushy. Optional: Save ½ cup of pasta water to add to the sauce.
Make the sauce. In a large skillet, melt the butter over medium-high heat. Cook the garlic in the butter until fragrant, about 1 minute. Whisk in the flour so there are no lumps and cook for 1 minute, constantly stirring. Stir in the pumpkin puree.
Simmer. Slowly pour in the milk and cream, whisking constantly to prevent lumps. Reduce the heat to medium-low and bring to a simmer. Stir in the cheese, salt, and pepper. Taste the sauce to check if more seasoning is needed.
Toss. Add the cooked pasta to the pan and toss with the sauce. If needed, adjust the thickness of the final sauce by adding hot pasta water.
Enjoy. Serve immediately with a little fresh grated nutmeg on top.