Preheat your oven to 400°F (200°C). Place the eggplant slices on two large baking sheets, brush both sides lightly with 2 tablespoons of olive oil, and season with salt and pepper.

Roast the eggplant in the preheated oven for 20-25 minutes until tender and lightly browned, flipping halfway through. Remove from the oven and reduce the temperature to 350°F (175°C).

While the eggplant roasts, heat the remaining 1 tablespoon of olive oil in a large skillet over medium-high heat. Add the chopped onion and cook for about 5 minutes until softened and translucent.

Stir in the ground beef and minced garlic, breaking the meat apart with a wooden spoon. Cook for 7-8 minutes until the beef is fully browned and no pink remains, then drain any excess fat.

Pour in the crushed tomatoes, cinnamon, oregano, salt, and pepper. Reduce the heat to low and simmer for 15 minutes until the sauce thickens and the liquid mostly evaporates, then remove from heat.

In a separate saucepan, melt the butter over medium heat. Whisk in the flour and cook for 1 minute until lightly golden and bubbly to form a roux.

Gradually whisk the warm whole milk into the roux, stirring constantly to prevent lumps. Cook for 5-7 minutes until the béchamel sauce is thick and creamy enough to coat the back of a spoon.

Remove the saucepan from the heat. Quickly and vigorously whisk in the Parmesan cheese and egg yolks until the sauce is perfectly smooth and glossy.

Assemble the moussaka in a 9x13-inch baking dish by laying down half of the roasted eggplant slices in an even layer. Spread the meat sauce completely over the top, then layer the remaining eggplant slices.

Pour the creamy béchamel sauce evenly over the top layer of eggplant, smoothing it to the edges with a rubber spatula.

Bake at 350°F (175°C) for 35-40 minutes until the top is golden brown and bubbling around the edges. Let the moussaka rest for at least 15-20 minutes before slicing to help the layers set, ensuring perfect pieces when serving.