While the oven is doing its job, you can do the sauce. To do this, fry the flour with butter in a saucepan, stirring all the time, until you get a beige color. Remove from the heat, pour in all the milk at once, mix with a whisk until smooth. Return to medium heat, cook, stirring, for 5 minutes. Add the crumbled mozzarella, remove from the heat and let the cheese melt. Season the sauce with salt and nutmeg.