Boil a pot of water and cook the macaroni according to package instructions.
Meanwhile, melt the butter over medium heat in a skillet or large saucepan.
When the butter starts making a sizzling noise, add the onion and cook it for about 3 minutes, stirring occasionally, until the onion starts to soften and turn transparent.
Add the flour, and stir it around for about 1 minute, until the flour just starts to brown. This combination of butter and flour is called a "roux" and it's how we're going to thicken the cheese sauce.
Whisk in the milk and keep whisking over medium heat until the sauce begins to thicken.
Stir in the canned pumpkin and the cheese.
At this point, after you've boiled and drained the macaroni, you can stir the macaroni and the sauce together. Enjoy!