Preheat your oven to 375°F (190°C) and prepare the baking dish by greasing with olive oil.

Slice the eggplants into 1/2-inch rounds, sprinkle with salt, and let sit for 30 minutes.

In a skillet, heat olive oil and sauté diced onion for 5-7 minutes until translucent. Add minced garlic and cook for another minute.

Layer the ingredients in the greased baking dish starting with eggplant, followed by tomatoes, sautéed onion, and garlic.

Top the last layer with grated cheese, then bake uncovered for 35-40 minutes.

Let the gratin rest for at least 10 minutes before serving.