Start by putting the butter in a small pot/saucepan and melt it.

I usually set it to 9 or 10 on my stove.

Add flour to the pot and whisk it well. Keep at it until you have a smooth paste and let the process take 2-3 minutes to ensure the flour is well cooked through, so it doesn't taste like flour.

Add 1/3 of the milk and whisk it well. Don't start too hard, or you'll splash it all over the kitchen (speaking from experience :-)).

Gradually add the rest of the milk while stirring it well. The goal is to eliminate all lumps, and the sauce should thicken.

Season with salt, pepper, and grated nutmeg.

When the sauce tastes just right, turn off the stove and let it sit for 30-60 seconds.

Add the grated cheese (and mustard if you like) and stir it well into the sauce. If the burner is still on, there's a chance the cheese will become stringy.

The sauce should have thickened quite a bit; if it's too thick, just add a little milk.

If you want the sauce to be even creamier, add an egg yolk and whisk the sauce for another minute.

Enjoy.