Slice the eggplants into ¼-inch rounds and lay them on paper towels. Sprinkle salt over the slices and let rest for 45 minutes. Pat dry.

Heat 2 tablespoons of olive oil in a skillet over medium heat. Fry the eggplant slices for about 2 minutes on each side until golden. Repeat with all slices.

In a saucepan, heat 1 tablespoon of olive oil. Sauté chopped onion and minced garlic until fragrant. Stir in diced tomatoes, herbs de Provence, basil, salt, and pepper. Simmer for 20 minutes.

Preheat the oven to 350°F (180°C). Grease a baking dish with olive oil. Layer half of the eggplant slices, half of the tomato sauce, half of the cheeses, and repeat. Finish with remaining sauce and toppings.

Bake for 40 minutes until bubbling and golden. Broil for an additional 2 minutes for extra crispiness.

Let the gratin cool for 15 minutes before serving.