In a large skillet over medium heat, add olive oil and sauté one chopped onion until translucent. Incorporate one pound ground beef, cooking until browned. Stir in one cup of tomato sauce and a teaspoon of ground cinnamon. Cover and simmer for 30 minutes.

Boil salted water in a large pot and cook 12 ounces of pastitsio noodles for 8-10 minutes until al dente. Drain and rinse with cold water. Mix with one beaten egg.

In a medium saucepan, melt 4 tablespoons of unsalted butter. Whisk in 4 tablespoons of flour for 2 minutes. Gradually add 2 cups of whole milk, whisking until thickened. Stir in two beaten eggs off the heat.

Preheat oven to 350°F. Grease a 12 x 18-inch baking pan. Layer half of the cooked noodles, half of the meat sauce, and 1 cup of Kasseri cheese. Repeat layers and top with béchamel.

Bake for about 1 hour until golden brown. Let rest for 10 minutes before slicing. Serve warm.