Pour the ayran in a bowl. Add the baking soda, eggs, oil and salt. Stir well and add the crumbled feta cheese.
In another bowl, cut the onions and boiled eggs. Stir well.
Pour 1/2 of the mixture into a buttered form/tray.
Distribute the filling on top and cover with the remaining mixture. Bake at 392°F (200 °C) for about 40 min. or until your toothpick comes out dry when you poke it in the middle.
Take the pirog out of the oven and leave it to cool a bit. Then invert it onto a cooking grid and leave it until fully cooled.
Transfer to suitable plates and serve.