Heat a dry non-stick pan and add the lamb, breaking it up with a fork. Cook until brown, then tip the meat into a bowl. Heat the oil in the pan and fry the courgettes and aubergines until golden, about 6-8 mins.
Return the meat to the pan and add the lentils, half the oregano and the passata. Crumble in the stock cube. Fill the passata jar to about half full with water (about 340ml), swirl and add to the pan. Simmer for 25-30 mins until the lentils are tender, adding more water if you need to. Season to taste.
Heat the grill to high. Mash together the rest of the oregano with the yogurt, feta and some seasoning. Pour the lamb mixture into 1 large or 4 individual ovenproof dishes, spoon over the topping, scatter on the crumbs, then grill for 3-4 mins until bubbling. Serve with salad and flatbreads, if you like.