Melt the butter in a saucepan over medium heat then brush a 9x13inch sheet pan with butter. Preheat the oven to 200 degrees C, or 392 degrees F.
In a medium/large bowl whisk in the eggs, yogurt, oil, and sparkling water.
In another bowl mix in the all-purpose flour, cornflour, salt, and baking powder.
Mix in the dry into the wet ingredients until everything is smooth and incprporated. You don't have to be careful to overmix the batter because it doesn't make any difference.
Crumble or grate 2/3 or around 80 g of feta cheese with your hands and add it to the batter mix again. Add the blanched spinach along with the feta cheese.
Pour the batter into the pan, grate the remaining cheese, sprinkle the seeds on top and transfer it to the oven.
Bake at 200C, 390F for the first 15 minutes then lower the temperature down to 180C 355F and bake for additional 15-20 minutes.