Make the meat sauce one day ahead. In a large pot, add a little olive oil and heat over medium heat. Add the chopped onions and sprinkle with a generous pinch of salt, cook until onions are just translucent. Add the ground beef and cook and stir until no longer pink, breaking it up with a wooden spoon as it cooks. Add the 2 cans of tomato sauce and cinnamon, stir and bring to a simmer. Season with salt to taste. Let meat sauce stew for 3 hours, yes, for 3 hours with a lid on, slightly askew. Stir occasionally. Let cool and then refrigerate overnight.
Bring sauce to room temperature or warm gently before layering the pastitsio. Grate the kasseri cheese and set aside, you want at least 4 cups.
Cook noodles in boiling salted water until al dente (I cook a minute or two less than package directions). Drain noodles and when cool enough to handle, put them in a large bowl and mix one beaten egg into the pastitsio noodles with your hands.
Preheat oven to 350º. In a lasagna pan, or other large pan, drizzle a little olive oil in the bottom or coat lightly with cooking spray. Put all of the noodles which were tossed with egg in the bottom of the pan and arrange evenly. Sprinkle with a third of the shredded cheese. Using a slotted spoon, cover the noodles and cheese evenly with all or most of the meat mixture, leaving room for the béchamel layer on top. Sprinkle another third of the cheese over the meat layer. You now have noodles, cheese, meat, cheese layered so far.
Make the béchamel sauce. In a heavy bottomed large pot, melt 1 stick of butter. Add the flour to the melted butter and whisk to combine well and cook, stirring constantly for a minute or two. Slowly add 5 cups of milk, whisking the whole time. Cook and whisk until it just starts to boil, when it starts to bubble. Turn off heat.
In a separate bowl, beat 3 eggs and 1/2 cup milk with a hand mixer. Add this mixture to the pot, slowly, whisking the whole time.
Put back on medium-high heat and cook and whisk until thick and bubbly. When at the desired consistency, cut the heat and let the béchamel sit for a few minutes.
In a small sauce pan, melt 1/2 stick of butter. After the béchamel has rested a few minutes, pour it over the meat and cheese layer, spreading evenly over the top. Sprinkle the remaining cheese over the béchamel. Lastly, drizzle or spoon the melted butter on top of the cheese, this is what will brown the top of the pastitsio. (Okay, Flora says to use a stick of melted butter. Too much I think, especially for my pan size. But that is what she says to use.)
Place pan on a baking sheet and then into preheated oven and bake for 1 hour, until browned and center is hot. If not browned enough, after 1 hour, turn on broiler and cook another 3 - 5 minutes, watching carefully until top is browned.
Let pastitsio rest for 10 minutes before serving.