Put a large pan on a medium heat and add the olive oil. Peel and finely chop the onion. Add to the pan and cook gently for 5 minutes.
Peel and chop the garlic cloves. Add to the pan and sizzle for 30 seconds, then tip in the beef mince. Cook, stirring regularly, until browned.
Pour in the passata. Add the nutmeg, cinnamon and bay leaf and a pinch of salt and pepper. Turn the heat to low and let it simmer for at least 45 minutes. It should be a thick sauce, but if it looks like it’s getting too dry add a splash of water.
For the bechamel topping
Melt the butter in a saucepan over a medium heat. Add the flour and whisk to form a thick paste.
Turn the heat off, then gradually add the milk, whisking all the time. When all of the milk has been added, turn the heat back on and continue whisking until the sauce thickens.
Turn the heat off and add the nutmeg and yolks - whisk. Leave until you’re ready to layer, reserving the Parmesan for the end.
For the pasta
Cook the pasta according to packet instructions. Drain.
Crumble the feta and mix in with the pasta.
When the pasta has cooled slightly, gently stir in the egg whites then tip into a large ovenproof baking dish. Use a wooden spoon to pat the pasta down to create an even layer. Leave until you are ready to bake.
Baking the pastitsio
When you’re ready, preheat the oven to 180 degrees C.
Pour the meat sauce on top of the pasta in the baking dish. Pat down to create a smooth layer.
Pour the bechamel on top of this (be careful, it will be a tight fit!) and sprinkle the Parmesan on top.
Bake in the oven for 30-45 minutes until golden brown.