Put the spinach into your biggest pan with the water, over a low heat. Put the lid on and allow it to steam for until wilted - taking the lid off and giving it a good stir every minute or so. Drain the spinach and roughly chop it.
In a large frying pan, saute the onion in olive oil until soft. Add the garlic, chilli flakes, lemon zest salt and pepper, and cook for another 2 minutes. Add the chopped herbs and cook until they’ve wilted, then switch off the heat. Add the chopped spinach and mix through, then leave to cool for 30 minutes.
Once the filling has cooled, beat the eggs and add them to the spinach,. Crumble the feta in and mix it all together. Preheat the oven to 200C /190C fan.
Brush the inside of the bundt tin with melted butter and sprinkle the sesame seeds in the bottom. Lay a sheet of filo with one corner in the middle of the bundt tin (half of the sheet will be hanging over the outside edge of the tin for you to fold in at the end) then brush that with melted butter. Be generous with the butter as you layer the filo around the tin, each one half covering the last but clockwise so that last sheet half covers the first. Brush butter over each sheet as you layer it up.
Spoon the filling into the bundt tin and then fold each sheet of filo back over the tin to seal it, layering with butter brushed over each sheet. Fold the inside edges in as neatly as possible, brush the whole top with butter then cook for 45 minutes. Take the pie out of the oven and carefully flip it onto a flat baking sheet. Tuck in any bits of pastry and bake for another 15 minutes out of the tin, until golden brown all over. Leave to stand for 10 minutes before cutting into slices to serve.