Salt cod is big in Portugal and Spain but in Scotland we were preserving fish in salt hundreds of years ago, with Shetlanders exchanging it for goods including wine, corn and textiles with European traders visiting the islands. The same principle applies with this week's recipe although I have cut down the amount of salt used for a fillet of fish that goes perfectly with the peppery watercress soup.

Salt cod with watercress soup

Serves 4

250g rock salt

500g cod fillet, skinless

1 onion

2 garlic cloves

125g unsalted butter

175g baking potato

750ml vegetable stock

200g watercress

Set the oven to 180C/160C fan/gas mark 4.

Start the recipe the day before. Place half of the salt in a deep tray with the cod on top then sprinkle the rest of the salt over the fish and set aside for 15 minutes. Next brush off the salt and lightly rinse the cod in cold water. Now place it on to a clean cloth on top of a tray and place in the fridge for 24 hours to dry.

The next day, when the cod is dry, cut it into four even pieces. Now make the soup. Peel the onion and garlic and slice them finely. Place a medium saucepan on a medium heat and add the butter. When the butter is foaming add the onion and garlic and season well with salt. Reduce the heat and cook until translucent and soft. While the onion is cooking peel and slice the potato finely then add it to the pot and stir well.

Next add the stock and bring it to a simmer. Cook the soup on a medium heat for 15 minutes before adding the watercress and cooking for a further two minutes. Now transfer the soup to a liquidiser and blitz until smooth. Taste and season the soup with pepper and more salt if required.

Place the soup into a shallow, wide pan and heat it to a simmer, then turn the heat off and add the cod portions. Leave to one side to cook in the residual heat for 15 minutes, making sure the cod is submerged. To serve, place each piece of cod in a shallow bowl and pour the soup around it.