Boil the spinach in salted water, strain it and cut it. Clean and cut the fish into cubes, boil it in the spinach broth.
Grate the onions, peel and slice the tomatoes, then add them to the boiling broth.
Add the spinach and tomato paste, leave the soup to finish boiling. Thicken it with the toasted flour, season with a little grated horseradish and the butter.
Upon serving, sprinkle with dill.