Heat 2 tsp of the oil in a large flameproof casserole over a medium-low heat and fry the onion for 5 minutes until beginning to soften. Add the garlic and fry for a further 1-2 minutes. Increase the heat to medium and add the lamb to the casserole, the fry, stirring, for 5 minutes until browned all over.
Stir in the cumin and coriander and cook for 1 minute, then add the chopped tomatoes, pomegranate molasses, stock and cinnamon stick. Bring to the boil, then reduce the heat and simmer gently for 30 minutes until the lamb is tender and the liquid has reduced slightly. Add half the chopped mint for the last 10 minutes of cooking and preheat the oven to 200°C/gas mark 6.
Discard the cinnamon stick and spoon the lamb mixture into a 1½ litre pie dish. Loosely scrunch each sheet of filo pastry and arrange on top of the filling to cover completely. Drizzle over the remaining oil and scatter over the sesame seeds.
Place the pie dish on a baking sheet and bake for 20 minutes until the pastry is crisp and golden. Stir the rest of the mint into the yogurt with the cucumber and serve with the pie.