This is a recipe for a classic sweet and sour herring.
This recipe starts with salted herring fillets.
Soak the herring in water a few hours to take out the salt. Change the water a couple times. It is best if you soak it over night.
Remove the herring from the soak water and place it in a container that has one cup of water, 1 cup of wine vinegar, and 1 cup of sugar. Place in the refrigerator and marinate the herring in this combination at least overnight.
Serve as you wish. There is no need to garnish his herring or add any other sauce.
Here are some good videos and below that links to many Polish food recipes.