Melt the 1/2 cup butter in a pan over medium heat. Stir in the flour with a wooden spoon. Cook's Note: This is an important moment, as you have to slowly toast the flour without burning it. This will help you lose the flowery taste.
Warm up the milk and gradually ladle into the pot with the butter-flour mixture, whisking constantly while bringing the mixture to a boil. Reduce the heat, and simmer for about 15 minutes. Season the sauce with freshly grated nutmeg, salt, and pepper.
If the sauce is too thick, add a little more milk, if too runny, return to the heat and add a pat of butter mixed with an equal amount of all-purpose flour. The most important thing though is: besciamella should not taste floury. If you think your sauce is ready, but you can spot a hint of "flouriness" when you taste it, think again, and keep on cooking it for a few minutes more.