Meanwhile, make the rue by melting the butter in a medium sized pan, once bubbling remove from the heat and quickly stir in the flour to make a thick paste (add more if you need to)
Add tiny bits of milk to this at a time, each time stirring vigorously to make sure it is well incorporated before adding some more
Once you have more of a liquid consistency, return the mixture to a gentle heat and slowly add the rest of the milk, continuing to stir
Bring the mixture to a rolling boil, continuing to stir until it starts to thick
Add a handful of the cheese and stir until it is all melted
Add the nutmeg and some black pepper
Transfer your steamed cauliflower to a ovenproof dish, pour over the cheese sauce and if you like, top with a little more cheese before placing in the oven
Remove when the mixture is bubbling and the top is turning brown (about 15 minutes)