Preheat your oven to 400°F and line a baking sheet with parchment paper.
Slice the eggplant into half-inch thick rounds, brush both sides with half of the olive oil, and roast for 15 minutes until softened.
While eggplant roasts, heat the remaining olive oil in a skillet over medium heat and sauté the diced onion and minced garlic until translucent.
Add the ground lamb to the skillet, breaking it apart with a spoon, and cook until fully browned.
Stir in the tomato puree, cinnamon, oregano, sea salt, and black pepper, then let the meat sauce simmer for 5-7 minutes.
In a small mixing bowl, whisk together the Greek yogurt, egg whites, and grated parmesan cheese until the mixture is smooth and combined.
In a small oven-safe baking dish, layer half of the roasted eggplant slices, followed by the lamb mixture, and then the remaining eggplant.
Pour the yogurt custard over the top layer of eggplant, spreading it evenly to the edges.
Reduce oven heat to 375°F and bake the moussaka for 20-25 minutes until the custard top is set and lightly golden brown.