In a large, heavy bottomed saucepan, bring salted water with a drizzle of olive oil to a boil. Cook the pasta, stirring often until al dente. Drain and set aside.
In the same pan, melt the butter over medium heat. Add the flour, 1 tbsp at a time, whisking after each addition. This is called a roux. Cook it down for four minutes, until the roux is a light caramel color.
Slowly (SLOWLY) drizzle in the warmed milk, whisking vigorously the whole time. I like to start with about 1/3 cup of milk, stir, 1/2 cup of milk, stir, and then very slowly add the rest. Cook for 10 minutes, whisking often.
Reduce the stove temperature to low, and add the shredded one handful at a time. Whisk to fully incorporate the cheese before adding any more. As soon as all the cheese is fully melted, immediately remove the pan from heat and add the pasta. Stir to combine.