Disassemble herring on a fillet. A detailed article on how to cut herring and cut it into pieces will help you. Cut the onion into half rings.
For marinade, mix mustard, sugar, salt, olive oil and vinegar. Mix well.
Put the fish and onions in the jar one by one, add the marinade, close the lid and shake it so that the marinade is distributed over all pieces of herring.
Leave to marinate in the refrigerator overnight.
According to this recipe, pickled herring is very tasty, fragrant and tender, onions and mustard go well with herring. For cooking, you can use fresh or salted herring, if you found only the second version of the fish, then salt does not need to be added. Serve as an appetizer on rye bread with onions, as well as an addition to mashed potatoes.