For the sake of keeping it manageable lets stick to Northern Europe (England, France, Scandinavia, Germanic Countries) and from the time of the Newfoundland fishing fleets forward ( Early 1500s).
Lutefisk in Norway and Sweden. You take cod, soak it cold water for a few days, then let it soak in lye of all things, then soak it cold water again to make it edible. This article goes over the origins, and the first mention of Lutefisk is in 1555.