Preheat the oven to 400 degrees.
Turn the stovetop burner to medium heat and add 1 tablespoon of olive oil to the same pot you used to cook your pasta.
Add your diced onions, sprinkle with salt, and cook for 5-10 minutes, stirring as needed.
Add your chopped sage and stir to combine. Cook for 1-2 minutes.
Sprinkle the mixture with the AP flour and cook for 2-3 minutes while continuously stirring.
Pour in your cup of milk, stirring to thoroughly combine with the flour mixture. You do not want to see any lumps.
Add the 4 ounces (half a small can) of pureed pumpkin. Stir into the mixture.
Small handful by handful, add the grated cheese, stirring to completely incorporate it before adding the next.
Once all the cheese is added, taste your sauce. Add salt, pepper, and any additional sage as needed. Add the nutritional yeast if you are using it.
Turn off the burner and remove the pot from the heat.
Add the pasta back to the pot and stir to completely coat the noodles with the sauce.