Slice the aubergines lengthways. Aim for ¼ inch thick slices. Add them to a large bowl and add 2 teaspoons of salt. Toss them with your hands making sure the salt is evenly distributed between all the slices. Set aside for 30 mins.
Preheat the oven to 180 C and get out a large baking tray.
Meanwhile make your sauce and grate the cheese. Sauce: blend the tomatoes (skins and all) to a smooth purée.
Add 1 tablespoon of olive oil to a saucepan and fry the onions until browned. Add the garlic, chilli flakes and oregano and cook for a minute.
Pour in the tomato purée and season with salt and pepper. Cover and cook until the sauce reduces down to almost a third of its original volume–about 15 mins. Stir in the basil and set aside to cool slightly.
Squeeze the aubergines to rid them of their excess moisture. They’ll be very pliable after salting.
Add the flour to a plate. Heat just enough olive oil in a pan to coat the bottom.
Dredge the aubergine slices in the flour until coated on both sides. Tap off the excess flour and cook until golden on both sides. Add a bit more oil if necessary. Do this in batches adding a bit more oil for each batch. Set aside.
To assemble: 1) Start with a layer of tomato sauce on the bread. 2) Add a layer of aubergine slices on the sauce, overlapping slightly if necessary. 3) Top with the two cheeses. Repeat one more layer in the same order.
Lay them out on the baking tray. Bake until the cheese melts through–about 10-12 mins.
Serve as a sandwich with another slice of bread on top, or as an open-face sub without it. Slice down in half if necessary. (Toast the top slice of bread separately if using.)