Preheat the oven to 220°C/200°C fan. Slice the eggplant into thin disks and place them on a lined baking tray (use two trays, if you're cooking for 3 or 4). Drizzle or brush with oil and sprinkle with salt. Roast for 20-25 min until browned.

Meanwhile, peel and chop the onion and garlic. Heat a pan over a medium heat with a drizzle of oil. Once hot, add the beef, onion and a pinch of salt. Break the meat up as you go with a spatula. Fry for 5 min with a pinch of salt. Add the garlic and cook for 2 min further. 

Add the tomato paste and plum tomatoes along with all their juices, the measured water, thyme, bay leaves, cinnamon stick and black pepper to the pan. Bring to a simmer, reduce the heat to low, cover with a lid and cook for 15 min (or longer, time permitting). Finally, remove the bay leaves and cinnamon stick. Check the seasoning. 

Meanwhile, heat a non-stick pan or saucepan over a medium heat. Once hot, add the butter and flour and cook, stirring, for 1 min or until a sandy paste has formed. Gradually whisk in the milk and simmer for 3-5 min or until the sauce has thickened. Season with a pinch of salt.

Layer the roasted eggplants and the beef sauce in a baking dish. Pour the bechamel over the top. Grate the Parmesan and crumble the feta over the top. Bake in the oven for 15 min or until nicely browned. 

Let the pastitsio cool for 5 min before slicing it. Serve the baby spinach alongside.