Pasta: 

1. Pour 10 cups of water into a large pot that has a lid. 
2. Add 4 tablespoons kosher salt to the water, cover the pot with its lid, and bring to a boil over high heat. (shred the cheeses while you wait for the pasta to boil)
3. Add the elbow macaroni to the boiling water, stir, and bring back to a boil over high heat. 
4. As soon as the pasta comes back up to a boil, set a timer for 1 minute less than the lowest recommended time for the pasta. 
5. Drain the noodles in a colander placed in the sink, toss with 1 tablespoon of butter to prevent from sticking and leave alone while you make the sauce. 

Cheese sauce:

1. Shred the cheddar and mozzarella using the large holes of a box grater, set aside.
2. Melt the butter in a large pot over medium heat. 
3. Make a roux by adding the flour to the butter and cooking, stirring frequently with a wooden spoon, until the roux is bubbling and semi solid/semi liquid, 3-4 minutes. You want to cook the flour a bit but not add any color by overcooking.
4. To test the roux, rub a small amount in between your thumb and index finger (be careful, it’ll be hot). The roux should feel sandy, not velvety smooth. 
5. Switch to a whisk, and add a little bit of the milk to the roux at a time, stirring well in between each addition. 
6. Continue to add the milk and whisk until smooth. 
7. Let the milk come to a simmer over medium heat, stirring frequently, until the sauce thickens, 5-6 minutes. 
8. Add 2 teaspoons kosher salt, the cayenne pepper, nutmeg, and dijon mustard to the sauce. 
9. Add the shredded cheese to the milk sauce (bechamel) in small batches, stirring with a wooden spoon in between each addition to melt the cheese. Remove from heat. 
10. Taste the sauce for seasoning. Add up to 2 teaspoons more kosher salt to the cheese sauce if necessary. 
11. Toss the cooked macaroni with the cheese sauce and serve immediately.