Rolling mops is an acid herring fillet that has been rolled up. The herring fillet is provided with onions and pickles. The whole is rolled up and secured with a pricker and inlaid in a blend of herbs. The herring forms the basis for the production of the rolling mops and is a derivative of the sour herring. Rollmopsen are made of herring caught after June. After the herring has been hom or spit, these are too lean to be sold as Dutch New. Herring is a versatile fish and is eaten a lot. The herring is a silver-colored fish and has a dark blue/green backing. Depending on the season, fresh herring are usually frozen on board for the manufacture of the salt herring or any fresh landing should be put in vinegar.