1) Preheat the oven to 200°C/gas mark 6.
2) Heat about 1/3cm of olive oil in a very large frying pan over medium heat. When the oil is almost smoking, add several slices of aubergine and cook, turning once, until they are evenly browned on both sides and cooked through, about 5 minutes. Be careful, it splatters! Transfer the cooked aubergine slices to kitchen paper to drain. Add more oil, heat, and add more aubergine until all the slices are cooked.
3) Meanwhile, in a small bowl, mix together the ricotta, egg, cream, 20g of the Parmesan, pinch salt and pepper.
4) In each of 2 individual gratin dishes, place a layer of aubergine slices, then sprinkle with parmesan, salt and pepper and spoon half of the tomato sauce. Next, add a second layer of aubergine, more salt and pepper, half the ricotta mixture, and finally 1 tablespoon of grated parmesan on top.
5) Place the gratins on a baking sheet and bake for 25 to 30 minutes or until the custard sets and the top is browned. Serve warm.