Wash the fish and cut it into small pieces. Melt the butter in a saucepan, add sliced vegetables and sauté them until onion is transparent. Add curry powder and flour, and stir until well blended. Then add fish and fish stock, and stir until boiling. Season with salt and freshly ground black pepper. Lower heat and simmer gently for 10 minutes. Lift out most attractive fish pieces and reserve them to serve in the soup. Simmer soup for 1½ hours longer, skimming when necessary.
Strain soup through a fine sieve, rubbing through