In a deep bowl, mix the flour with the salt, sugar, and yeast. Pour in the warm kefir and melted butter. Knead the dough.
Set the dough in a warm place for one hour.
Punch down the risen dough and divide it into 8 pieces. Shape into balls and place on a baking sheet.
Meanwhile, fry the onion in half-rings and slice the feta.
Press a hollow into each bun with the bottom of a glass. Brush the buns with beaten egg and fill the hollows with feta and onion.
Bake for 25-30 minutes in a preheated 200°C oven. Let the buns cool - and enjoy!