If using frozen fish, first thaw it in cold water. Once thawed, start preparing the fish.

Cut the fish into several centimeter-long pieces.

Transfer the fish to a saucepan and cover with water (adjust the amount to submerge all pieces). Season with a pinch of salt and bring to a boil. Reduce the heat and cook until the fish is properly thermally processed, about 5-10 minutes depending on the size of the pieces.

While the fish is cooking, prepare the other ingredients. Wash, peel, and grate the carrots, parsley, and a piece of celery on a coarse grater. Wash, clean, and finely chop the leek.

Heat the oil in a saucepan. Add the leek and fry for a few minutes, stirring occasionally.

Then add the carrots, parsley, and celery. Mix and reduce the heat. Fry under a lid until soft, about 10-15 minutes, stirring occasionally.

Towards the end, add the tomato passata and concentrate. Season to taste with salt and pepper. Mix thoroughly and simmer for a moment.

Arrange the fish and vegetables alternately in a glass or ceramic dish.

Set aside to cool. After cooling, cover and refrigerate. It's best to prepare the dish one or two days in advance for the flavor to intensify.

Serve cold or reheated.