In a large skillet over medium heat, sauté one finely chopped yellow onion with a pinch of kosher salt in olive oil until translucent, about 5-7 minutes.

Add 1 pound of ground beef, browning it thoroughly for 8-10 minutes.

Stir in 2 cups of tomato sauce and 1 teaspoon of ground cinnamon. Cover the skillet and let the sauce simmer on low for 3 hours, allowing flavors to meld.

For maximum flavor, refrigerate the sauce overnight.

Bring a large pot of salted water to a boil and cook the pastitsio noodles until al dente, about 1-2 minutes less than the package instructions.

Drain the noodles and immediately transfer them to a large bowl. Toss the hot noodles with one beaten egg, ensuring they are well coated.

Allow the mixture to cool slightly while you prepare the next steps.

Preheat your oven to 350°F (175°C). In a large baking dish, layer half of the cooked noodles on the bottom.

Sprinkle about 2 cups of grated kasseri cheese over the noodles, then evenly spread half of the prepared meat sauce on top.

Repeat the layering process with the remaining noodles, cheese, and meat sauce.

In a saucepan over medium heat, melt 4 tablespoons of unsalted butter. Whisk in 4 tablespoons of all-purpose flour, stirring constantly to create a roux.

Gradually whisk in 3 cups of whole milk and continue whisking until the mixture thickens, about 5-7 minutes.

Remove from heat and mix in 2 beaten eggs until incorporated.

Pour the béchamel sauce over the layered pastitsio, ensuring it cascades down the sides.

Sprinkle any remaining kasseri cheese on top, followed by a drizzle of melted butter.

Place the baking dish in the preheated oven and bake for 1 hour, or until the top is golden brown.

Once baked, allow it to rest for about 10 minutes before slicing.

Cut into generous squares and serve warm.