Syrup
In a saucepan, add the water, mandarine juice, sugar and mandarine peel. Stir and bring to a boil. Reduce the heat and simmer for 5 minutes. Remove from heat and let the syrup cool completely.
Filo pastry
Take one filo sheet and lightly scrunch it to create air and volume. Shape it into a loose ball and place it in a baking pan. Repeat with the remaining sheets.
Bake in a preheated oven at 180°C (356°F) for 10 minutes. Remove the pan, turn the filo pieces over, and continue baking for another 10 minutes, until golden and crispy.
Allow them to cool, then break them into small pieces by hand.
Mandarine mixture
Wash the mandarines well and cut them in half. Remove the seeds and place them in a blender. Remove the white pith from the mandarine peels and add the zest to the blender. Blend until smooth.
In a large bowl, beat the eggs with the sugar until light and fluffy. Add the milk, sunflower oil, fresh mandarine juice, mandarine purée, baking powder, baking soda and vanilla. Mix well.
Add the broken filo pieces and gently mix until fully incorporated.
Pour the mixture into a rectangular baking pan 23 × 33 cm (9 × 13 in). Smooth the surface and bake at 180°C (356°F) for 40 minutes.
Remove the mandarine pie from the oven and slowly pour the cold syrup evenly over the hot cake. Let it absorb the syrup completely, cool well, and serve.