Whisk vigorously 1/2 cup (120ml/4 ounces) milk into the roux until incorporated into a paste.
Keep adding milk a little at a time to incorporate the milk while whisking to keep lumps from forming.
Bring the sauce up to a simmer by increasing the heat to medium.
After you achieve a slow simmer, turn down the heat and gently cook the sauce for 20-30 minutes while skimming off any skin that forms.
When the sauce thickens (coating the back of a spoon) and the starchy taste is cooked out, add seasonings and taste.
Seasonings to taste (pinches of salt, pepper/white pepper, mustard powder, freshly grated nutmeg)
Turn off the heat and allow the sauce to cool, stirring during the cooling process to prevent a skin from forming on the surface.