Preheat the oven to 190°C / fan 170°C / gas 5
Cook the onion in the olive oil, until soft and clear
Add the spinach, add a squeeze of lemon juice and touch of salt and pepper, once the spinach has thawed and cooked in with the onion - leave to cool.
In a bowl break up the block of feta, and add the cooled down spinach and onion mix – mix together thoroughly
Prepare the pastry – Remove the filo pastry from its packaging and cover with a damp tea towel (keeping it covered as you work). Take a sheet at a time, brush all over with melted butter then lay in the tin, so it covers the base and overhangs the sides. Take another sheet, brush with butter and lay on top, overlapping the sheet already in place, one you have 3 layers, add your spinach and feta mixture
Now add more sheets to top the pie, with butter brushed in-between each
Once you’ve used all the pastry – score the top into squares, how you will end up serving each piece
Have a final brush of butter, sprinkle some nigella and sesame seeds over the top and bake for around 25-30 minutes or until the pastry is golden brown