Beat 4 eggs. Mix them with the feta cheese. Add the yoghurt and sprinkle the salt.
Smear the tray with oil. Arrange 2-3 phyllo pastry sheets and smear them with the resulting mixture.
Portion the mixture so you have enough to smear every 3 sheets, about 4 times total. Do not put any on top of the final sheets.
Cut the whole thing up into squares. Heat the oil. Pour it in a thin trickle into the cracks.
Beat 1 egg in another container and pour in the carbonated water. Pour this over the phyllo pastry and spread it out evenly.
Bake at about 338°F (170 °C) on just the lower element at first and once it fluffs up, bake with the upper one too.