Preheat your oven to 400°F and slice the eggplant into 1/2-inch thick rounds.
Lightly brush the eggplant slices with olive oil, season with a pinch of sea salt, and roast on a parchment-lined sheet for 15-20 minutes until tender.
While the eggplant roasts, finely dice the onion and mince the garlic.
In a non-stick skillet, sauté the onion and garlic until translucent, then stir in the cooked lentils, tomato puree, cinnamon, oregano, salt, and pepper.
Simmer the lentil mixture for 5 minutes to allow the flavors to meld, then remove from heat.
In a separate bowl, whisk together the non-fat Greek yogurt and the egg until the mixture is completely smooth.
In a small oven-safe baking dish, layer half of the roasted eggplant, followed by the lentil mixture, and then the remaining eggplant slices.
Pour the yogurt and egg custard over the top, sprinkle with crumbled feta cheese, and bake at 375°F for 20-25 minutes until the topping is set and beautifully golden.