How to cook
Separate the egg whites from the yolks, beat the yoghurt and milk with the yolks. Put a pot on the stove with the butter and just a tiny bit of oil.
Once it heats up well, braise the flour in it until golden, add the mixture with egg yolks, season with salt and pepper and stir nonstop until thickened.
Lower the heat to a minimum, let simmer slowly about 5 min. while stirring to prevent it from sticking.
Turn the stove off and wait for it to cool (it will become quite thick once it does). In the meantime, beat the egg whites to snow and then mix them carefully with the mixture once cooled.
This topping really is unique and stays on top of the moussaka (as it should). It's about 3/4″ (2 cm) thick, although of course this depends on the tray you'll be baking as well. Try it and it's the only way you'll ever cook your moussaka.