In a large pan or medium saucepan, melt butter over medium heat
Once butter is completely melted, add flour and cook until floury flavor is cooked off and roux begins to smell nutty, about 2 minutes. Do not let mixture brown.
Add 1 cup milk, then whisk until completely combined. Roux will seize up. This is typical. Repeat, adding about 1 cup at a time, until mixture is liquidy and has no lumps. Then add remainder of milk
Let cook, whisking often, until mixture thickens and can heavily coat the back of a spoon.
Once desired thickness is achieved, take mixture off heat and add salt, white pepper, and nutmeg.
Mix in cheese 1 handful at a time, whisking until cheese is completely melted before adding next handful. If additional heat is needed, put burner on low and whisk constantly until cheese is fully incorporated. Remove from heat.
Boil pasta in heavily salted water until it is 3 minutes from al dente. Drain, and add to cheese sauce. Mix to combine. Set aside and let cool completely.