In a medium saucepan, melt the butter over medium heat.
Add the flour and whisk continuously for about 1-2 minutes to form a roux.
Gradually pour in the milk, whisking constantly to prevent lumps.
Cook the mixture, stirring frequently, until it thickens (about 5-7 minutes).
Season with salt and pepper to taste.
Add a pinch of nutmeg if desired for added flavor.
Remove from heat and let it cool slightly before using.
Cover the surface with plastic wrap if not using immediately.
Use in lasagna, moussaka, or as a base for cheese sauce.
Adjust seasoning before serving as needed.