Heat 1 tablespoon of the oil in a frying pan, add the onion and fry for 5 minutes, stirring until just beginning to brown. Add the garlic, courgettes, mushrooms and peppers and fry for 2–3 minutes.
Stir in the tomatoes, rosemary, sugar and seasoning, bring to the boil then cover and simmer for 15 minutes. Tip into a shallow ovenproof dish, leaving enough space to add the aubergines and topping.
Rinse and dry the aubergines. Heat 2 tablespoons of the oil in a clean frying pan and fry half the aubergine slices until softened and golden on both sides. Arrange, overlapping, on top of the tomato mixture. Repeat with remaining oil and aubergines. Allow to cool then chill until required.
Stir the eggs, yogurt, nutmeg and a little pepper together in a bowl, cover and chill until required.
When ready to serve, stir the yogurt mixture once more then pour over the aubergines. Sprinkle with the feta and bake in a preheated oven, 180°C (350°F), Gas Mark 4, for 30–35 minutes until piping hot. Spoon on to plates and serve with garlic bread and salad.