Place six slices of toast in a deep baking tray and cover them with half the cheese and half the ham, as well as half the spinach. Repeat all these steps for the second layer.
With the help of a glass, cut a hole in the center of six slices of toast. These toast slices now go in the baking tray as the third layer. Place a raw egg in each of the circular holes in the toast. You can use the leftover toast circles for other purposes.
Prepare a roux with the butter and flour and pour in the milk while stirring constantly. Season the sauce with a little salt, pepper and nutmeg. Then mix in the mozzarella and the parmesan until the sauce reaches an even, creamy consistency. Pour the cheese between the slices of toast.
Place your giant breakfast sandwich in the oven for 20 minutes at 340°F with the heat set to above and below.