Preheat the oven to 200°C and keep filo pastry covered with a damp cloth.
Fry the sliced onion in half of the olive oil for 5-6 minutes until soft.
Wilt the spinach in the remaining oil, then drain and cool in a colander.
Remove onions from the heat and allow to cool on a plate.
Layer five filo sheets on a baking tray, brushing each with melted butter.
Season cooled spinach with nutmeg, salt, pepper, sultanas, onions, and pine nuts.
Mix in crumbled feta and enough beaten egg to moisten the mixture without making it runny.
Spoon the mixture onto the pastry base, leaving a 4cm border.
Fold the pastry edges inward to partially enclose the filling.
Layer the remaining five filo sheets on top, brushing each with melted butter.
Score the top and bake for 45-50 minutes until golden-brown.
Serve the sliced pie with a rocket salad.