Bring a pot of water to a boil and cook the macaroni according to the package directions.
When the pasta is done drain the water and rinse it with cold water until it is no longer hot. (If you don’t cool it well enough all of the pasta will stick together while you’re making the cheese sauce and then you’ll have big clumps of uncheesy pasta) Set aside until your cheese sauce is ready.
Making the Roux
Once you put the water on the stove dice the onion into super tiny pieces and fry it in 1 Tablespoon of the butter, stirring occasionally. You want the onion to turn translucent/golden.
Set the onion aside and put the rest of the butter in a pot and melt on medium/low heat. Once the butter is melted add the flour and mix to form a paste. This is called a roux.
Cook the roux until it turns slightly brown.
Start adding the milk a little at a time, mixing it in to the roux. This should turn into a thick paste.
Add the can of cheddar soup, the black pepper, and 1/2 of the bag of shredded cheddar and mix until everything is melted and you’ve got a thick cheesy sauce.
The Mac & Cheese
Grease a 9X13 pan with a little butter so the mac & cheese doesn’t stick. If cooking right away preheat the oven to 350.
Add the onions to the cheese sauce and mix until combined.
Add the cooled macaroni and mix until well coated.
Pour the mac & cheese into the pan. If making ahead cover pan and refrigerate until you’re ready to bake. If eating right away place the pan into the oven and bake for 20 minutes.
Sprinkle the other half of the bag of cheese on top and bake for another 5-10 minutes or until the cheese on top is as crispy as you like. If you want a more browned and crunchy top broil for 3-5 minutes.