Slice the eggplants lengthwise into 3/4 to 1-inch thick slices. Place the slices in a colander, sprinkling each layer with kosher salt as you go.

Set a plate on top of the eggplant slices and weigh it down with a heavy can. Let drain in the sink for 45-60 minutes to draw out excess moisture and bitterness.

After draining, thoroughly pat the eggplant slices dry with paper towels.

While the eggplant drains, prepare the tomato sauce. Heat 2 tablespoons of olive oil in a heavy saucepan over medium heat.

Add the chopped onion and cook until soft and translucent, about 5 minutes.

Add the minced garlic and cook for just 1 minute more, being careful not to burn it.

Add the canned tomatoes, salt, pepper, and red pepper flakes (if using). Simmer over low heat for about 30 minutes until the sauce thickens.

Stir in the freshly chopped basil and remove from heat.

Preheat the broiler. Brush the eggplant slices with the remaining olive oil on both sides.

Broil the eggplant slices until lightly browned on both sides, turning once.

Preheat the oven to 350°F (175°C).

In a large baking pan, spread a thin layer of sauce to cover the bottom.

Arrange a layer of browned eggplant slices without letting them touch each other (to maintain crispiness).

Top each slice with a spoonful of sauce and sprinkle generously with both mozzarella and parmesan cheeses.

Continue layering until all ingredients are used up, finishing with sauce and cheese on top.

Bake uncovered for 35-40 minutes until bubbly and the top is beautifully browned.

Let rest for 5-10 minutes before serving to allow the layers to set.