Put the lentils in a saucepan with the tomatoes, garlic, oregano and nutmeg. Pour in the stock. Bring to the boil, then reduce the heat and simmer for 20 minutes until the lentils are tender but not mushy, topping up with extra stock as needed.
Meanwhile heat the oil in a frying pan and lightly fry the aubergine and onion, until the onion is soft and the aubergine is golden on both sides.
Layer the aubergine mixture and lentil mixture alternately in an ovenproof dish.
Make the topping. In a bowl, beat together the egg, cheese and nutmeg with a good dash of salt and pepper. Pour over the moussaka and cook in a preheated oven, 200°C (400°F), Gas Mark 6, for 20–25 minutes.
Remove from the oven and leave to stand for 5 minutes before serving with salad leaves.