Cut the butter into pieces (all the same size for them to cook uniformly) and place them in a medium pan. Melt over medium heat.
Add all the flour and stir energetically with a wooden spoon until completely amalgamated.
Remove from heat and let the temperature decrease for a couple of minutes.
Add the milk, return to the stovetop and bring to a boil over medium heat, stirring continuously.
Add the salt, pepper, and nutmeg and mix.
Cover and cook on the lowest heat for 15-20 minutes (if you see sides browning, remove immediately).