First make the syrup by mixing all ingredients together in a saucepan. Bring to a boil, lower the heat to medium and simmer uncovered for 15 minutes. Let syrup cool completely. Transfer to the fridge until needed.
Next you need to dry the phyllo pastry. Some recipes suggest you leave the phyllo soft as it is, but I find that the only way to distribute the custard evenly my drying the phyllo sheets. You can do this in two ways. You can separate the phyllo pieces and lay them one by one on a table to dry. This can take anything from 10 minutes to 6 hours. Especially if the weather is humid. I sometimes leave them on my kitchen counter overnight. But if you are in a hurry you can dry them one by one in a 100° C oven, for 10 minutes. After your phyllo sheets are dry, crush them with your hands to small uneven pieces. Put them in a bowl until needed.
Preheat oven to 180° C. Beat eggs and sugar in the bowl of a mixer until light and fluffy. Add orange juice, orange zest, oil, yogurt, vanilla and salt. Beat until combined. Add baking powder and beat lightly to mix.
Brush a 23 x 33 cm baking pan with sunflower oil. You must use a baking pan for your “Portokalopita”. I find that a glass or ceramic pan does not distribute heat as quickly and evenly as a metal one and this might leave your pie uncooked in the centre or not as fluffy as the edges.
Put your custard mixture in a large bowl. Take handfuls of crushed phyllo and throw it in the custard bowl. Fold into the custard making sure, phyllo pieces are not stuck together and the custard is distributed evenly. Don’t put all the crushed phyllo in the custard at once. Do this in several doses.
Spread mixture in prepared pan. If you want to decorate with orange slices, these must be very - very thin (1 mm thick max). You can also leave the pie plain and decorate with orange slices before serving. I quite like the roasted orange effect.
Bake pie for 30 minutes until dark brown. Reduce oven to 160° C, cover pie loosely with foil to prevent from burning (do this as quickly as possible, trying not to remove the pie from the oven). Bake 15 minutes longer. A cake tester or a skewer inserted in the centre of the pie must come out clean.
As soon as it comes out of the oven, ladle the cold syrup all over the hot Portokalopita. It may seem like a lot, but it’s not. The pie will absorb everything eventually and become this really moist, sweet, absolutely wonderful pie with tiny pockets filled with orangey syrup. Leave to cool completely before slicing and serving plain or with a dollop of vanilla or mastic ice cream. It’s divine!