Preheat your oven to 400°F and line a baking sheet with parchment paper.
Slice the eggplant into 1/2-inch thick rounds, brush lightly with olive oil, and sprinkle with a pinch of salt.
Roast the eggplant for 15-20 minutes until the slices are tender and slightly browned, then remove from the oven.
While eggplant roasts, sauté the diced onion and minced garlic in a non-stick pan over medium heat until translucent.
Add the ground lamb to the pan, breaking it up with a spoon and browning it thoroughly; drain any excess fat.
Stir in the tomato puree, cinnamon, oregano, sea salt, and black pepper, then let the mixture simmer for 5 minutes.
In a small mixing bowl, whisk together the Greek yogurt, egg, and parmesan cheese until smooth to create the topping.
In a small oven-safe baking dish, layer half of the roasted eggplant, followed by the lamb mixture, and then the remaining eggplant.
Spread the yogurt and egg mixture evenly over the top layer of eggplant.
Lower the oven temperature to 375°F and bake the moussaka for 20-25 minutes until the custard topping is set and golden brown.