Heat a generous drizzle of olive oil in a large, heavy-based casserole over a medium heat. Add the diced onion with a pinch of sea salt and cook gently for 6–8 minutes until softened and translucent. Add the beef mince, breaking it up with a wooden spoon, and cook until evenly browned.
Stir in the passata and ground cinnamon, season generously with salt and freshly ground black pepper, then bring the sauce to a gentle simmer. Partially cover the pan with a lid and leave to cook slowly for around 3 hours, stirring occasionally, until rich, thick and deeply flavoured. Remove from the heat, allow to cool completely, then refrigerate overnight. This resting time is highly recommended, as it allows the flavours to mature and develop beautifully.
When you're ready to assemble the pastitsio, remove the meat sauce from the fridge and allow it to come back to room temperature. Meanwhile, grate the cheese and divide it into three equal portions for layering.
Bring a large saucepan of generously salted water to the boil and cook the pastitsio pasta until just al dente, around 1–2 minutes less than the packet instructions. Drain thoroughly and leave to cool for a few minutes before transferring to a large bowl. Add the beaten egg and gently toss until every piece of pasta is lightly coated. This helps the pasta hold together when sliced.
Preheat the oven to 180°C (160°C Fan/Gas Mark 4). Lightly grease a large baking dish with olive oil or butter. Spread the pasta evenly across the base before scattering over the first third of the grated cheese. Spoon the meat sauce evenly over the pasta using a slotted spoon, leaving any excess liquid behind. Finish with the second layer of cheese, creating generous layers of pasta, cheese, slow-cooked meat and more cheese.
To prepare the béchamel, melt the butter in a large saucepan over a medium heat. Whisk in the flour and cook for around 2 minutes, stirring continuously to remove any raw flour taste. Gradually pour in the milk, whisking constantly until the sauce is perfectly smooth and free from lumps. Continue cooking until it thickens and just begins to bubble.
In a separate bowl, whisk together the eggs with the extra milk until smooth. Slowly pour this mixture into the béchamel while whisking continuously. Return the pan to a gentle heat for another minute or two until the sauce becomes thick, glossy and wonderfully velvety, then remove from the heat and leave to rest for a few minutes.
Pour the warm béchamel evenly over the meat layer, smoothing the surface with a spatula until completely covered. Scatter over the remaining cheese, then drizzle with the melted butter to encourage a beautifully crisp, golden finish.
Place the baking dish on a baking tray and bake for 1 hour, or until the top is deeply golden and the filling is bubbling around the edges. For an even richer colour, place the dish under a hot grill for the final 3–5 minutes, watching carefully to prevent it from catching.
Remove the pastitsio from the oven and leave it to rest for 10–15 minutes before serving. This final pause allows the layers to settle, ensuring each slice holds its shape beautifully while remaining wonderfully moist and full of flavour.
