Delicooks

Cod is one of Spain’s most popular dishes, with each region boasting its own recipes. Its widespread use is thanks, chiefly, to its easy preservation. “Bacalao al pil-pil” is undoubtedly the most famous cod dish and is named after the noise produced by oil in contact with the drops of gelatinous liquid given off by the cod as the oil heats. “Bacalao al pil-pil”
is the  sauce that results when the gelatin from the cod emulsifies with the oil.
Ajoarriero is a dish popularized by muleteers on the trade route linking Bilbao to Zaragoza and was salt cod eaten by the camp fire with a mixture of sauteed garlic, tomato, chili and green pepper. Crayfish were also added.

A bit of history.
In the nineteenth century, the port of Bilbao was the main trade center devoted to cod, with all kinds of cod from far and wide passing through the docks. Cod is thus inextricably linked to the Basque port. There’s also the interesting anecdote about one Jose Maria Gurtubay, importer of cod, who placed an order for "one hundred or one hundred and twenty cod". Jose Maria wrote the order in numbers, but through some misunderstanding, received one million and one hundred and twenty cod. Jose Maria was almost suicidal when the city was besieged during the course of the First Carlist War. It was Jose Maria’s cod which then fed the besieged city, making him one of the richest men in Bilbao.