Parboil the pasta. Drain and put in the baking pan. You should toss it around in some butter, oil, or margarine to keep it from sticking. Then add a generous amount of grated cheese and toss. You also have the option of placing half the pasta in the dish, filling the middle layer with meat sauce, and adding more pasta on top. You also have another option of tossing the pasta and the meat sauce together and leaving it like that. Do whatever your heart desires. You may also toss in some egg whites. Beat them a bit with a fork, and then toss them around with your hands into the pasta mixture. They just do this so they don't waste the eggs. It really doesn't make it tastier or anything.
For the meat sauce in a large pot, sauté the onion, garlic, and parsley in the oil until the onions turn translucent. Then add tomato paste (optional) and stir for a few seconds. Season with salt and pepper.
Add chopped meat. Stir, let it release some juice. You could, at this point, add a splash of red wine for change in taste but it's really not necessary.
Add the chopped tomatoes, cloves, and cinnamon, and let the tomatoes release some juice. Stir.
Add the tomato sauce or crushed tomatoes and stir. Taste for salt.
Add a cup or two of water, bring to a boil, reduce heat, and simmer on low for an hour and a half or until there is no water left in the pot.
Now, as I said before, you have the option of layering the meat sauce, or mixing it in with the pasta.
For the béchamel, in a saucepan on low heat, melt one stick and a half of butter.
Gradually add flour and whisk until the roux forms a gunky ball and you can't stir anymore.
Gradually add 6 cups of milk and the bouillon (if you want it). I'd say even 5 and 1/2, because I like my sauce really thick. Stir occasionally and make sure you whisk away the flour caught around the edges with a rubber spatula and whisk again. Once the sauce thickens, remove from heat. Add grated cheese, the egg yolks (slowly so they don't curdle while whisking very fast), the nutmeg. Add a pinch of salt (if necessary)and pepper. If you like creamy sauce, make it with the full six cups of milk.
Make sure your pasta and meat sauce are at an even level in the pan, and then pour in the béchamel on top. Even it out with a spatula.
You may top with more grated cheese, nutmeg, cinnamon, or even breadcrumbs.
Bake in a 350 degree oven for about 40 minutes or until the top turns golden brown.
Lastly and most importantly, let the pastitsio rest for an hour or two before eating. The béchamel has to set and solidify a little bit. If you don't wait, the sauce will gush out.