Prepare the meat sauce by cooking onions in olive oil, adding ground beef, then stirring in tomato sauce and cinnamon. Simmer for three hours.

Cool and refrigerate the meat sauce overnight.

Cook pastitsio noodles in salted water until al dente, then mix in one beaten egg.

Preheat oven to 350ºF and assemble layers with noodles, cheese, and meat sauce.

Make béchamel sauce by melting butter, adding flour, and whisking in milk until bubbling.

Combine eggs and milk, then incorporate into béchamel while whisking.

Thicken béchamel on medium-high heat until desired consistency is reached.

Pour béchamel over layered ingredients and sprinkle remaining cheese on top.

Bake for about one hour until golden brown and bubbly.

Let Pastitsio rest for ten minutes before slicing and serving.