In a large pot over medium heat, add a little olive oil. Add the diced yellow onion and a generous pinch of kosher salt, then cook until the onions are just translucent. Add the ground beef and cook, stirring and breaking up the meat with a wooden spoon, until no longer pink. Stir in the tomato sauce and ground cinnamon, season with salt to taste, and bring to a simmer. Cover with a lid slightly askew and let the meat sauce stew for 3 hours, stirring occasionally. Once finished, let the sauce cool, and refrigerate overnight. I like to make the sauce a day ahead to deepen the flavors.
Before assembling the pastitsio, bring the meat sauce to room temperature or gently warm it up. Grate the kasseri cheese (or other cheese blend) to yield at least 4 cups, and set aside.
Bring a large pot of salted water to a boil. Add the pastitsio noodles or mostaccioli and cook until al dente, about 1–2 minutes less than the package directions suggest. Drain the noodles, then once they’re cool enough to handle, place them in a large bowl. Beat one egg and mix it thoroughly into the noodles with your hands.
Preheat the oven to 350ºF. Lightly coat a lasagna pan or large baking dish with a drizzle of olive oil or nonstick spray. Evenly spread all of the noodles mixed with egg (from Step 3) into the bottom of the prepared pan. Sprinkle one-third of the shredded cheese (from Step 2) over the noodles. Using a slotted spoon, evenly spoon all or most of the meat sauce (from Step 1) over the cheese, leaving enough space for the béchamel layer. Top the meat layer with another third of the shredded cheese, so your layers are: noodles, cheese, meat, cheese so far.
In a heavy-bottomed large pot, melt the stick of unsalted butter over medium heat. Add all-purpose flour and whisk to combine, stirring constantly for about 1–2 minutes. Gradually add 5 cups whole milk, whisking constantly to prevent lumps. Continue cooking and whisking until the sauce just starts to boil and bubble, then turn off the heat. In a separate bowl, beat 3 eggs with 1/2 cup whole milk using a hand mixer. Slowly pour the egg-milk mixture into the pot, whisking constantly. Return the pot to medium-high heat and whisk until thick and bubbly. Once the béchamel has reached the desired consistency, cut the heat and let it rest for a few minutes. For extra lusciousness, I always make sure to whisk vigorously to keep the béchamel smooth and silky.
In a small saucepan, melt the remaining 1/2 stick of unsalted butter. After the béchamel sauce (from Step 5) has rested for a few minutes, pour it over the meat and cheese layers (from Step 4), spreading it evenly to cover the surface. Sprinkle the remaining cheese (from Step 2) over the béchamel, then drizzle or spoon the melted butter on top (you can use less than the full amount if you prefer). Place the assembled pan on a baking sheet and bake in the preheated 350ºF oven for 1 hour or until browned and bubbly in the center. If additional browning is desired, switch on the broiler and cook for another 3–5 minutes, watching carefully. I sometimes add just a bit less butter on top for a golden, crisp crust without excess richness.
Remove the pan from the oven and let the pastitsio rest for 10 minutes before serving. This helps the layers set and makes slicing easier. I always find that a short rest ensures perfect squares when serving, and it keeps the dish deliciously moist.
