Make the pie crust and set it in the fridge to rest for at least ½ hour.
In the mean time preheat the oven to 200°C or 400°F.
After a rest in the fridge take the pie dough roll out to about 2 millimeters in thickness. Line the quiche pan, dock the dough then let it rest for at least ½ hour.
Then blind bake it for 20 minutes. This will help keep the crust from getting soggy from the spinach filling. To blind bake simply line the tart shell with baking paper and fill with beans or weights made for this purpose. This stops the shell from shrinking during the baking.
On a medium heat put the frozen spinach in a pot just large enough to hold it with a little water, cover and bring to the boil. Remove and strain. Squeeze out all the excess water with the back of a large spoon, it shouldn't be very wet.
Chop it coarsely if its whole leaf spinach.
In a little olive oil and butter saute the onions over a medium heat until they are soft and pale in color, about 5 minutes.
Now combine all the ingredients in a large bowl. Mix well and taste for salt and pepper. Be careful with the salt, Feta is quite salty on it's own.
When the pie shell has been partially baked and cooled down a little, pour the spinach mixture into the shell making sure that the Feta is well distributed.
Bake with the rack placed in the middle of the oven for about 35 minutes.
Serve with a tomato salad and some crispy Italian bread or a baguette.