Preheat the oven to gas 6, 200ºC, fan 180ºC. Heat a grill or griddle pan until hot. Brush the aubergine slices with 2 tablespoons of oil and grill both sides until golden.
Heat the remaining oil in a large pan and cook the onion and garlic for 2 minutes before adding the mince. Cook until browned all over, then stir in the flour, oregano, bay leaf and tomato purée. Mix, add the milk and stock and bring to the boil. Turn the heat down, simmer for 20 minutes, stirring occasionally.
Meanwhile, make the white sauce. Melt the butter in a pan, add the flour and cook, stirring, for 2 minutes. Remove from the heat, gradually whisk in the milk, then stir over a low heat for 5 minutes. Remove from the heat again, season and stir in the egg yolks and nutmeg.
Layer half of the aubergine slices in the bottom of an ovenproof dish and top with half the tomatoes. Pour over the mince, then layer on the remaining aubergines and tomatoes. Top with white sauce and Parmesan and bake for 20-25 minutes.
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