Peel the potatoes and cut them into quarters lengthwise. Cut two onions into rough cubes. Sauté in a large Reindl together with the potato pieces and a tiny bit of olive oil, season with pepper and season with salt. Add the rosemary needles and place in the stove at 180 °C form. From time to time turn the potatoes to the other side.
Rinse the fish, scale it and remove the gills with scissors, season with pepper and season with salt. Roughly dice the remaining onion and sauté in a frying pan with the olives. Fill the fish with the onion-olive mixture and wrap tightly with spaghetti. Sauté on all sides in the frying pan and set aside.
Once the potatoes are cooked for fifteen minutes, move them to the side and add the fish. Cook everything together for another 20 minutes at 180 °C, then fillet and serve with the potatoes.