First gut the fish and remove the scales. Cut the potatoes in a symmetrical way and slice the lemons.
Saute the potatoes in a deep pot with olive oil. Score the onions slightly from the underside and pour them into the pan stirring for 2-3 minutes.
Pour in the juice of one lemon. Add a little salt, pepper, chili pepper (optional), pour 600 ml. water and allow the mixture to boil for about 8-10 minutes.
Then place the fish in the pot and simmer for about 10 minutes without shaking.
Remove the pot from the fire, pour in the juice of the other lemon and shake the pot carefully to prevent the fish from crumpling. Let the soup cool down for 10 minutes before serving.
At a platter serve the fish with the potatoes and onions and season them with olive oil. Serve the soup in deep dishes.