Place butter in microwave safe container and melt on medium heat for 20 seconds.
Add flour and stir well. Put in microwave on medium heat for another 20 seconds. At this stage you might be a little scared of what the mixture looks like but don't worry it will be fine.
Add half a cup of milk and stir well. Add another half a cup and stir well. Then place in microwave on medium heat for 30 seconds and stir.
Add half a cup of milk and stir well. Put in microwave on medium heat for 1 minute. The mixture should start to thicken. Mix well.
Add the remaining milk, stir well. Using a whisk can help at this point. Put back in microwave on medium for another minute. Whisk. If not thick enough put back in on medium for another minute and then whisk again. Keep doing this until its thick enough. Don't worry if it looks lumpy with a good whisk it will come together fine. Lastly add in some grated fresh nutmeg. If you find the mixture has become too think add in some extra milk 1 tbsp at a time.
Of course you can always do this on the stove top in the usual way. I just like using the microwave because I find it foolproof and easy.