Bacalao is fish, traditionally cod fish, that is cured by dry-salting it. In Portugal it is used to make savory seafood stews, and it is also used in Spain and France to make the creamy spread called brandada or brandade.
Although usually made with cod, it can also be made with other mild, flaky white fish such as haddock. Cod has been overfished and is endangered in some parts of the world, so you may want to go with an alternative fish.
The method couldn't be simpler:
Prepare the Fish
Start with fillets of a mild, not too oily fish. Haddock or flounder will work as well as cod.
Rinse the fish fillets under cool water. Pat them dry with a clean dishtowel or paper towels.
Salt the Fish
Spread a 1/2-inch thick layer of medium grain sea salt or kosher salt on the bottom of a glass or stainless steel container: I suggest avoiding plastic containers to cure the fish in because some plastics contain bisphenol A (BPA) that can be risky for your health.
Lay the fish fillets in a single layer on top of the salt so that none of the pieces are touching. Spread another 1/2-inch layer of salt on top of the pieces of fish. If you have more fish to salt cure, place it on top of the second layer of salt, and then completely bury it in a third layer of salt. Continue like this in alternating layers of salt and fish until all the fish is used.
Finish with a layer of salt. It is important that the fish be completely buried in salt.
Cover the fish and salt and store it in a very cold but not freezing place (like your refrigerator) for 48 hours.
Dry the Fish
After salting it for two days, rinse the cured fish fillets under cold water. Pat them very dry with a clean dishtowel or paper towels.
Wrap the salt-cured, rinsed and dried fish in cheesecloth. Place it on a rack set over a plate or baking dish, and return it to the refrigerator, uncovered. Let it dry out in the refrigerator for 1 to 2 weeks.
When it is dry and stiff, transfer the bacalao to containers and cover tightly, or wrap it in foil or butcher's paper. Store it in the refrigerator for up to 3 months, or in the freezer for at least a year.
To Use Bacalao (Salt Fish)
Bacalao needs to be soaked in water for 24 hours before you cook with it. During that time, change the water at least twice. This rehydrates the fish as well as removing some of the salt.