Wash the fish thoroughly, cut off the head and tail with a sharp knife and store it. Fillet the fish, cut into large cubes and keep the bones. Season fish with salt.
Peel and slice the onions. Clean, wash and cut the peppers into pieces. Simmer the fish bones, tail and head, onions, diced peppers and the tomatoes in 1 1/4 litre salted water for 30 minutes.
Pour fish stock through a sieve, stir in tomato paste and season with salt and paprika. Add the fish pieces and simmer at medium heat for another 15 to 20 minutes. Serve the soup in deep plates and garnish with cherry paprika to taste.