In a large pot, melt the butter over medium heat. Add the all-purpose flour and vigorously stir with a wooden spoon for about 2 minutes, creating a roux.
4 tbsp of butter, 4 tbsp of all-purpose flour
Pour the cold milk into the pot all at once. Switch to a wire whisk and mix well until the mixture comes to a gentle boil.
1 L of milk
Reduce the heat and let the sauce simmer for about 10 minutes, stirring occasionally to ensure it doesn't stick to the bottom of the pot.
Season the sauce with a pinch of nutmeg, 2 tsp of salt, and freshly ground black pepper. If you'd like, you can also add 3 tbsp of white wine for extra flavor.
1 pinch of nutmeg, 2 tsp of salt, 3 tbsp of white wine