Mix flour and salt. Roll the fillet portions in the resulting mixture, then fry in a frying pan with 40 ml of well-heated oil until golden brown (2 minutes on each side).
Heat the remaining oil in another pan and fry the onion over medium heat for 5 minutes. Add the carrots and fry, stirring, for another 7 minutes.
Place tomato paste in the pan, add salt, sugar, water and stir. Boil for about 5 minutes, then transfer half of the carrot mixture to a plate, add fish to the rest of the pan and cover it with the mixture from the plate.
Cover the pan with a lid and simmer everything together for about 40 minutes on low heat, then serve.