Preheat the oven to 180°C fan / 400°F / Gas 6 and grease a glass or ceramic 25cm pie dish with some melted butter or line it with baking paper.
Brush a filo sheet with butter and pull the two long sides towards each other in a pleated pattern.
Twist it into a spiral or snail shell shape and place it in the centre of the prepared dish. (If you prefer you can also use this method for the whole dish instead of following the next step.)
Brush another filo sheet with butter and bring the long sides together again in a pleated pattern and coil it around the pastry in the centre. Repeat with the remaining butter and filo until you have filled the dish. Don't over pack the filo as you want to have space for the custard. Place the pie dish into the oven for 15 minutes until the filo is nice and golden. Meanwhile make the custard.
Custard: In a medium bowl, add the whole eggs, egg yolks, vanilla and half of the sugar.
Add the milk, cream, lemon zest, salt, and the remaining sugar to a medium saucepan and bring to the boil. Whisk occasionally until the sugar has dissolved.
Once the sugar has dissolved, whisk the egg and sugar mixture for about 30 seconds until it becomes a little frothy. Then pour the milk in a steady stream onto the egg mixture whisking continuously.
As soon as the filo has partially baked, remove it from the oven and pour the custard over the top into all the crevasses.
Baking: Place it back in the oven for 18-20 minutes until the top is golden brown and the custard has puffed up.
Remove from the oven, sprinkle with the icing sugar and cinnamon while hot, and then allow it to cool completely.