2) 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 slices to paper towels 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, milk or cream, half the Parmesan, 1/8 tsp salt, and 1/8 tsp pepper.
4) Place a layer of aubergine slices in two individual gratin dishes, then sprinkle with Parmesan, salt and pepper and spoon over half the marinara sauce. Next, add a second layer of aubergine, more salt and pepper, half the ricotta mixture, and finally 1 tbsp 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.