Béchamel: Melt the butter in a saucepan, add the flour, and stir well until the flour is lightly toasted. Gradually add the milk, whisking constantly to prevent lumps. When the mixture starts to thicken, be careful not to let it burn or darken. Remove from heat and add the 3 egg yolks. Finally, add salt, pepper, nutmeg, and the grated graviera cheese, reserving 2 tablespoons for the topping.

Pasta: Cook the pasta 1-2 minutes less than the package instructions. Since the dish will bake in the oven, the pasta will cook further, so it should be al dente.

Meat Sauce: Sauté the onions in a little olive oil. After 3-4 minutes, add the garlic and continue to sauté over medium heat. Add the tomato paste and sauté briefly, then add the bouillon cube and ground beef. Cook until the meat is browned. Pour in the wine and let it evaporate. Add the crushed tomatoes, sugar, salt, pepper, and herbs. Simmer over low heat until all the liquid is absorbed.

Add 1/4 to 1/3 of the béchamel to the meat sauce and mix well.

Spread the pasta in the bottom of a baking dish and arrange evenly. Add the 2 beaten eggs, crumbled feta, and a little salt and pepper to the pasta, mixing gently to distribute.

Spread the meat sauce over the pasta, then top with the remaining béchamel.

Sprinkle the reserved cheese on top of the pastitsio, and if desired, add a few small pieces of butter.

Bake in a preheated oven at 350-400°F (180-200°C) for 40 minutes or until golden brown.