Take a large bowl and melt some butter in it for 45 seconds in the microwave. When it's melted, add the flour and whisk. Pour some milk on top of this mixture, then microwave, mix and add some more milk. Heat for 1 minute. Repeat until the sauce is thick, adding more milk and heating until you've reached the consistency you desire. Season with salt and pepper and a touch of freshly grated nutmeg.