1. Omelette: Whisk the eggs and baking powder together. Add the milk and whisk until well mixed. Season with salt and pepper.
2. Heat half the butter in a large non-stick pan. Pour half the omelette mixture into the pan.
3. Filling: Put half the red onion on top, followed by half the atchar and half the feta. Put the lid on the pan and fry over moderate heat for 5 minutes.