First, cook your elbow macaroni according to the package directions. You wanna get it tender but with a little bite and not overcooked. Drain and set it aside.
Now grab a saucepan over medium heat and melt the butter. Stir in the flour right away and cook for about 1-2 minutes. Keep stirring so it doesn't stick or burn.
Gradually whisk in the milk. Keep stirring until the mix starts to thicken up a bit. This will be your creamy base for the sauce.
Stir in the pumpkin puree next. Sprinkle in the salt, pepper, and nutmeg. Cook it all for another 2-3 minutes, stirring occasionally.
Next, toss in the cheddar and Parmesan cheese. Stir until everything melts smooth and the sauce looks creamy and welcoming.
Add your cooked macaroni back into the sauce. Stir it all together so every piece gets covered in that pumpkin-cheese goodness.
Serve it warm right away. Optional: Sprinkle some extra cheese or herbs on top.
Notes
For leftover storage, refrigerate in an airtight container up to 4 days. Add a splash of milk when reheating for best texture.