Before refrigeration was invented fish would go off really quickly. Those lucky enough to live on the coast could eat that day's catch. Those inland had to eat meat and two veg. And then one day salt (conveniently from the sea) was found to preserve fish. Not only that, it preserved fish and allowed it to develop its flavour. Salt cod (morue in French) was born, would last for ever and could be exported inland and overseas.
Salting food extracts the moisture which in combination with bacteria leads to spoilage. So salted food, in this case cod, needs rehydrating to soften it, reduce the salt content and make it edible.
The drier and saltier the cod the more soaking it needs. I soaked my piece here (bought from Garcia) for 18 hours and changed the water 4 or 5 times. Salt cod in London can be found wherever there are Spaniards, Portuguese or West Indians living in any number. So, the northern end of Notting Hill is good as is Brixton. Otherwise, it's difficult to find. The French adore it and every fishmonger in France worth his salt will have it on sale. It's a gift to mongers because it's practically the only fish that won't go off. In Portugal they even have shops that won't sell anything else. And in the Caribbean the daily white rum aperitif is usually accompanied by salt cod fritters (ti punch & accras de morue on the French islands).
The Portuguese have hundreds of recipes for bacalhau (bacalao in Spanish) but the French just have the one, brandade de morue. Brandade comes from an old verb "to beat" and once the rehydrated fish has been poached for a few minutes in milk and or olive oil it is skinned, deboned and beaten into an emulsion, often with added potato.
It's not a dish for those who like mild flavours. As the fish has been aged it has quite a strong flavour in a similar way that meat goes high when it has been hung. When I poach the fish I add whole garlic cloves and pepper. I use the poaching liquor to mash the boiled potatoes. The fish is much easier to skin and debone when it's still warm; if you do this when it has cooled down it really sticks to your fingers (after all, the first glues were made from boiled fish). A sprinkling of parsley at the end is all it needs though this is my embellishment; brandade in even the smartest Parisian brasseries usually comes in an earthenware crock and looks like, well, mash.
As brandade is quite strong it's a challenge to match with wine. Here, we started off with my favourite foodie beer (Meantime India Pale Ale) which copes with any strong food that is thrown at it (the style was invented for curry). And as brandade is salty it's good to have something thirst quenching. We continued with old fashioned white Rioja (Marqués de Murrieta Gran Reserva 1998), aged for 32 months in cask and with the guts and acidity to cope. I had leftovers the next day with another old fashioned Rioja, but this time a Tondonia rosado 1997 from López de Heredia, aged 4 years in barrel. It's not often one comes across a 13 year old rosé wine that is still drinkable but again lengthy barrel ageing makes these wines pretty indestructible and gives them an almost sherryish, big oxidised flavour that allows them to cope with extremes of salt, savouriness and spice.
I sometimes make brandade without potato and spread it on bruschetta. In this case, I would probably serve a Fino Sherry or Manzanilla Pasada.