Cut the onion into half rings, grate the carrots on a coarse grater, and chop the celery stalks into medium cubes. Fry the vegetables in vegetable oil until soft. Add spices, ketchup and sugar. Grate the tomato on a coarse grater. Mix everything and pour in the water. Stir and simmer all together for another 5 minutes over medium heat.
Cut the fish fillet quite large, add a little salt. Add to the vegetables. Gently mix so that the fish is all under the vegetable marinade. Cover the pan with a lid and simmer the fish for 20 minutes over medium-low heat. Do not stir the fish more during stewing. Ready to serve fish with your favorite side dish.