For the filling, wash the spinach leaves and add to a pan with only the water that clings to them. Cover and cook gently until tender. Allow to cool slightly, then squeeze out the excess water and chop.
Soften the leeks and garlic in the oil over a medium heat. Add the mushrooms and
Line the prepared flan tin with one-third of the filo sheets, brushing each sheet with olive oil and leaving an overhang to fold over the filling.
Spread half of the filling into the lined flan case and press down evenly. Cover with half of the remaining filo sheets, again brushing each sheet with olive oil. Spread the rest of the filling on top and fold the overhanging pastry over the top.
Finely shred the remaining filo sheets and scrunch them over the top, to enclose the filling completely.
Brush with olive oil and sprinkle with sesame seeds, then
Carefully remove from the tin, cut into wedges and serve.