Wash the eggplants, remove the stalks and slice lengthwise into approximately 0.7 cm slices. Lay on a large plate and salt lightly both sides. Peel the onion and dice very finely.
Heat 15 ml of oil in a pan, sweat the onions until translucent, then sprinkle with sugar and cook until lightly caramelised. Add the tomatoes.
Season the sauce with oregano, basil, salt and pepper and simmer over a low heat for about 15 minutes.
Meanwhile pat the eggplant slices dry with a piece of kitchen roll and brown lightly in oil, in batches.
Take out of the pan and drain on kitchen roll. Drain the mozzarella and slice thinly.
Then lay a layer of eggplant slices in a greased baking dish, add a layer of tomato sauce and sprinkle with 15 ml of Parmesan.
Continue in this way until all the eggplant slices are used up (use any remaining tomato sauce for another purpose). Cover with the mozzarella slices and sprinkle with the remaining Parmesan. Bake in a preheated oven (200°C) for about 30 minutes.