Melt your butter in a saucepan over medium heat
Make a light rough, mixing in the flour and browning it for 2-3 minutes. What you want is a very light, golden-yellow color. Stir it frequently to prevent from burning.
And here is the trick of making a smooth béchamel sauce: Use very hot milk, just below its boiling point. Pour it onto the rough at once, and whisk it fast.
Cook your béchamel sauce over very low heat, whisking it regularly for a few minutes. It's ready when it thickens and covers the back of a wooden spoon.
If by any reason you find that your béchamel became lumpy, just use an immersion blender and it'll smooth it out in no time.
Season with salt and grated nutmeg.