Wash the eggplants, dry them and cut them lengthwise in slices about 1/5 inch thick (5mm); before grilling, pat them down with some paper towel on both sides.
Heat up the pan on medium heat and sprinkle with a little olive oil (half teaspoon should suffice) or a few sprays of cooking spray; once the pan is warm, add the eggplant slices and allow to cook.
Cooking the eggplant should take about 3 minutes per side. Ensure the eggplant slices are thoroughly cooked as they may release some water in the bake, making it liquid and quite bitter.
To check whether the eggplant is cooked thoroughly, use a fork and if it pokes and removes easily, it is ready. As the slices cook, set the tender eggplant slices aside on a baking sheet and continue until the whole batch is done.
Prepare the tomato sauce
In a large saucepan, warm up the olive oil on medium heath and, when warm, Sautée the onion and garlic until they are soft and the smell of rawness has gone.
Next, add the tomato passata, half a cup of water, and, if you have, add a few basil leaves and allow the sauce to simmer on a low flame for 15-20 minutes, occasionally mixing. Once the tomato sauce is ready, it will be bright red in color and quite thick.
Layer the eggplant parmesan
Prepare the baking dish by covering the base with some tomato sauce, which will prevent the eggplant from sticking and burning at the bottom.
Next, lay the slices of grilled eggplant and top them with a bit of grated parmesan and mozzarella cheese and top with some more tomato sauce; once done, repeat the same process, but layer the eggplant in the other direction, perpendicular to the previous layer. Continue doing so until all of the eggplants have been used.
For the top layer, first spread the remaining tomato sauce and then top it with the cheeses; the eggplant casserole is now ready and it only requires baking.
Preheat the oven to 180 degrees Celsius (FAN), 350 Fahrenheit FAN, gas 6 or static oven 200 degrees Celsius or 400 Fahrenheit.
Place the eggplant parmesan on the middle shelf in the oven and bake for 20 minutes covered with a sheet of aluminum foil and then, 20 more minutes without it.
Should there be still quite a bit of moisture in the bake, cover it again with the foil, leaving a corner open, to allow the steam to escape; continue baking until the desired consistency is achieved.