Slice the onions, carrot and celery into thin rounds. Cut the potatoes into thick fingers. Heat the butter and olive oil together in a deep pan and in this toss all the vegetables over gentle heat for 10 min. Add the curry powder and stir the mixture over the heat for a few more minutes. Add the boiling water, the herbs, salt and pepper. Cut the fish into neat pieces and add to the soup. Simmer until the fish is tender. Lift the best pieces of fish from the soup and keep them hot with a little of the liquid. Reduce the remaining liquid for 15 min.—pour it through a sieve and press through the potatoes and scraps of fish. Add the wine, if used, and reheat the soup. Blend the flour and milk, stir this into the soup and boil it. Return the pieces of fish and add the cream to the soup, just at boiling-point.