Clean and wash the leek and cut into fine rings. Bring the stock to the boil and cook the leek rings for about 5 minutes. Wash the fish fillet, dab dry and cut into thin strips. Add chilli and cod strips to the hot stock.
Cover the fish and let it simmer for about 5 minutes. (Do not boil). Cut off the cress and add to the stock. Season to taste with a little salt and lemon juice. Arrange in deep plates.