Heat half the oil in a casserole pot over a medium-high heat and fry the lamb for 5-7 mins, or until deep brown all over. Cover with the stock, then add the cinnamon stick and bay leaf, and bring to a simmer. Cover and cook for 11/2 hrs, or until the lamb is tender and falling apart.
Heat the remaining oil and the butter in a frying pan, then fry the onions with a large pinch of salt for 15-20 mins, or until golden and sticky. Add the garlic and spices, and cook for 3 mins more. Remove the lamb from its stewing liquid (reserving the liquid), transfer to a chopping board and shred using two forks. Stir the shredded meat into the onion mixture, along with 50ml of the reserved liquid.
Put the spinach in a pan with 1 tbsp water, and cook for 3-4 mins, or until wilted. Wrap in a clean tea towel and squeeze out the excess liquid, or put in a fine sieve and press it out using the back of a wooden spoon. Roughly chop the spinach, then stir into the lamb and onion mixture along with the feta, mint, pine nuts and egg. Season.
Heat the oven to 200C/180C fan/gas 6, and brush a 22cm round springform cake tin with some of the melted butter. Layer four sheets of filo pastry across the tin, then brush with more melted butter. Spoon in the filling, and fold the overhanging pastry over the top. Scrunch up the remaining pastry sheets, put on top of the pie, and brush with the remaining melted butter. Sprinkle with the sesame seeds. Bake for 45-50 mins, or until golden and crisp. Will keep in the fridge for up to up to two days.