Peel the zucchini and cut into medium cubes, half-rings onions. Put in a frying pan and fry for a couple of minutes over high heat for a couple of minutes in vegetable oil. Add finely chopped garlic and medium-sized bell pepper to the vegetables. Leave the vegetables to fry for another couple of minutes.
The vegetables are browned, the fire is quieter. Three tomatoes on a coarse grater, the peel remains with us, we do not add it to the vegetables. Put the grated tomato, ketchup in a pan, mix. Pour in a glass of boiled cold water and add salt, sugar, pepper and vinegar to taste. Mix everything, heat to a minimum and simmer vegetables under a closed lid for about 15 minutes.
Cut the fillet into small plates or cubes, as you like, and add to the vegetables, mix. Leave to simmer under a closed lid for 10 minutes.