Fillet the fish. Boil the waste, including the head, with the roots into a strong broth.
In the butter, sauté the cleaned vegetables cut into strips with the onion cubes. Pour in the strained broth (approx. 1 liter) and simmer gently for approx. 15 minutes. Cut the fish into cubes (not too small!). Drizzle with lemon, season with salt and add to the stock with the vegetables. Let simmer for about 15 minutes. Season to taste and sprinkle with plenty of chopped parsley.
Serve hot!
Apply pure or with light mixed bread (cut into large chunks).