Matjes Herring are pickled herring made from young, female herring with silvery skin who haven't laid eggs yet.
They are caught in the North Sea at the end of May and beginning of June, while they still have some of their winter fat in them.
In the Netherlands, before processing, they must by law first be frozen to below - 49 F (-45 C) to kill any parasites. They are sometimes skinned, sometimes filleted, sometimes left whole. They then are cleaned, though parts of the intestine, and the pancreas are deliberately left in. They are then brined for five days (traditionally, oak barrels) then pickled in a solution of vinegar, brown sugar and spices. Sometimes a red liquid with sandalwood or red wine is added, giving them a reddish hue.
They are not smoked, dried or otherwise cooked; they are essentially "raw." Some people say one could consider them "cooked" by the marinating process, in the same way that some people consider the Mexican fish dish "ceviche" to be "cooked." In practice, though, ceviche isn't considered "cooked" by most health authorities, and thus the Dutch law requiring freezing of the herring first.
Matjes Herring are popular in the Netherlands, Sweden and in Germany.
Soak in water or milk for an hour first before using. Some people don't bother soaking them, feeling that they're not as salty as other preserved herring. Can be eaten straight up afterward.
"Matjes" means "maiden."
Sometimes called "soused herring."