Preheat the oven to 375°F.
In a large skillet over medium heat, cook the ground beef or lamb until browned.
Add the chopped onion and garlic, cooking until the onion is translucent.
Stir in the diced tomatoes, tomato paste, oregano, cinnamon, salt, and pepper. Simmer for 10 minutes, then remove from heat.
While the meat sauce simmers, sprinkle salt on the eggplant slices and let them sit for about 30 minutes to draw out moisture. Rinse and pat dry.
In a separate skillet, heat 2 tablespoons of olive oil over medium heat and lightly fry the eggplant slices until golden brown, about 3-4 minutes per side. Set aside.
In a bowl, mix the ricotta cheese, half of the mozzarella, Parmesan, eggs, milk, and nutmeg until well combined.
In a greased 9x13-inch baking dish, layer half of the eggplant slices on the bottom.
Spread half of the meat sauce over the eggplant, followed by half of the ricotta mixture.
Repeat the layers with the remaining eggplant, meat sauce, and ricotta mixture. Top with the remaining mozzarella cheese.
Cover the dish with aluminum foil and bake for 30 minutes.
Remove the foil and bake for an additional 15-20 minutes, or until the top is bubbly and golden brown.
Let it cool for 10 minutes before serving.