First wash the fish fillet and then cut it into cubes. Mix with a little lemon juice and season with salt and pepper.
Heat 1 tablespoon of oil in a pan, add the fish, fry briefly and then remove. Now sauté the onions, add the chopped vegetables and sauté for 5 minutes. Add tomato paste, then deglaze with the stock, add the fish again. Simmer everything together for another 5 minutes over low heat, season well with salt and pepper.