Heat the oil in a large frying pan. Add the onion and cook for 3 mins until starting to colour and soften. Add the garlic, aubergine and lamb, then cook for 5-10 mins until the mince is brown and the aubergine is softened.
Stir in the lentils, tomatoes, potatoes, oregano, cinnamon, some seasoning and the stock. Increase the heat and simmer for 10-15 mins until the mixture thickens.
Heat grill to high. Transfer the lentil and lamb mix to an ovenproof dish. Mix the Greek yogurt with the beaten egg, then pour on top of the lentil and lamb mix. Grill for 5 mins until the top is bubbling and golden.