In a pan, melt the butter
At medium heat, add the flour and quickly stir
Step by step, add the milk while stirring. Add a pinch of nutmeg and salt.
Add the spinach to the white sauce (or Bechamel) and mix it well.
Pour into a baking dish, and cover with the grated mozzarella. Grill it for 5 minutes at 450°F .
There you are !