Melt the butter in a saucepan over medium heat.

Once the butter is melted, add the flour to the saucepan. Stir continuously with a whisk and cook for about 1-2 minutes until the mixture (roux) turns a light golden color. Be careful not to let it brown too much.

Gradually pour in the milk while whisking constantly to prevent lumps from forming.

Cook the sauce, stirring frequently, until it thickens and reaches a smooth consistency, typically around 5-7 minutes.

Season the sauce with salt, pepper, and a pinch of nutmeg if using. Taste and adjust the seasoning as needed.