Before refrigeration was invented, this was the fish enjoyed in the interior of Spain, featuring in a huge repertoire of dishes. Salt cod can be bought from city markets, Mediterranean delis and some good fishmongers. It always requires desalinating before using. To do this, brush away all the large salt crystals and place the cod in a large bowl of clean, cold water. Soak in the refrigerator for 48 hours, changing the water four times at regular intervals during this period. Remove the cod from the water and discard the water, then use as recipe directs.