Cut the onions into thin slices and fry them in the pan and let them cool.
Stir the sour cream, egg, salt, pepper, baking powder and sugar with the hand mixer, add the flour and whisk again well. Cut the feta cheese into small pieces.
Line a round baking pan (26 cm) with parchment paper and pour the dough into the pan. Place the feta cheese and onions on top. Bake in the preheated oven on the middle rack at 200 degrees for 20-25 minutes.