Top and tail the aubergines and slice them across thinly. Rub the slices on both sides with fine salt and leave in an unplugged sink for half an hour to drain. Meanwhile, wash the leeks thoroughly and slice across in 1.25 cm/½ in pieces, using as much of the green part as possible. Melt the butter in a large, heavy-based sauté pan, add the leeks, coriander and cumin seeds and cook over a low to medium heat, stirring often, until the leeks are really soft but not browned. Season with sea salt and plenty of black pepper and leave on one side.
Rinse the aubergine slices to get rid of all the salt and then pat them dry with absorbent paper. Put some plain flour into a mixing bowl and dip the slices into the flour to coat lightly. Heat a little olive oil in a large frying pan over a medium heat. Fry the aubergine slices for a few minutes on each side, until they feel soft when pierced with a knife. You will probably have to cook them in two or three relays, adding more oil as necessary and putting the cooked slices on absorbent paper on one side.
Preheat the oven to 190°C/375°F/gas mark 5. Put a thin layer of olive oil in a fairly shallow ovenproof dish and line it with about half the aubergine slices. Then slice the mozzarella cheese thinly and lay half the slices on top of the aubergines. Spoon in the leek mixture and top with the remaining slices of mozzarella. Cover with the remaining aubergine slices, sprinkle with the grated Parmesan and drizzle with olive oil. Bake near the top of the oven for about 20 minutes, until browned in patches.