1) Arrange the eggplant slices on several baking sheets and sprinkle generously all over with salt. Set aside to let the bitter juices weep from the eggplant, about 1 hour.
2) Transfer the eggplant to a colander in the sink, and rinse well under cold running water. Transfer eggplant to a work surface and blot very dry with paper towels.
3) In a large bowl, whisk together the 1 1/2 teaspoons salt, breadcrumbs, oregano, thyme, and season with pepper.
4) Place the flour in a medium lipped plate or bowl. In another medium bowl, whisk the egg and milk together. Dredge an eggplant slice in the flour, then dip it in the egg, and finally dredge it in the breadcrumb mixture. Shake off any excess breading and transfer the eggplant to a baking sheet. Repeat with the remaining eggplant.
5) In a large straight-sided skillet, pour the oil to a depth of 1cm inch. Heat the oil over medium heat until it registers 200C on a deep frying thermometer. (The oil must be heated to 200C so that the breaded eggplant, when added, will drop the temperature of the oil to the proper frying temperature of 190C.)
6) Working in small batches, fry the eggplant slices, turning once, until golden brown, about 3 minutes per batch. Using tongs, transfer to a paper towel-lined baking sheet and season with salt to taste. Repeat with the remaining eggplant.
7) Preheat the oven to 200C/Gas 6. Lightly brush a 35 by 23 by 5cm baking dish with olive oil. Cover the bottom of the baking dish with 1/3 of the marinara sauce and arrange half of the eggplant over the sauce. Cover the eggplant with another 1/3 of the sauce. Scatter half of the Parmesan and half of the mozzarella over the sauced eggplant. Repeat with the remaining eggplant, sauce, Parmesan, and mozzarella. Bake until hot and just beginning to brown, about 30 minutes. Serve immediately.
8) For the quick marinara sauce: Heat the oil in a medium saucepan over medium-high heat. Saute the onion and garlic, stirring, until lightly browned, about 3 minutes. Add the tomatoes and the herb sprigs and bring to a boil. Lower the heat and simmer, covered, for 10 minutes.
9) Remove and discard the herb sprigs. Stir in the salt and season with pepper to taste. Use immediately, store covered in the refrigerator for up to 3 days, or freeze for up to 2 months.