When it starts to shimmer, then add in the flour. Whisk to combine. Allow to cook for about 30 seconds, stirring constantly.
Add in ¼ cup of the milk. Mix to combine.
Add in the remaining 1 ¾ cups of milk. Stir to combine.
Bring to a boil and cook until the sauce coats the back of a spoon.
Remove from the heat.
Stir in the parmesan cheese. Grate the nutmeg into the pot.
Add kosher salt and white pepper to taste.
Serve with your favorite pasta.
Notes
The sauce might look too thin when you pull it from the heat, but as it cools a little it will thicken up to be the perfect consistency. If it ends up being too thick for your liking, slowly add in a little bit of water or milk. Then check the seasonings again.