You can pickle any type of fish or fillet in a solution that consists of an equal amount or more of vinegar to water. Abundant, inexpensive, sustainable fish with short shelf lives, such as sardine and herring, are natural choices. Spices and aromatic ingredients are entirely up to you. Classic pickling solutions, such as those in Norway, Finland and Poland, the region where the dish originated, contain bay leaf, a few black peppercorns, partially boiled onions and a few teaspoons of sugar per pint. Add aromatic ingredients, such as garlic, fennel and chili peppers, and herbs, such as dill, to taste.
Heat 1 part kosher salt with 3 1/2 parts filtered water and stir until dissolved. Let the saltwater cool.
Place the fillets in a plastic food-container and cover them with the brine. Cover and store the fish in the refrigerator for 24 hours.
Rinse the fillets and cut them into strips or bite-sized pieces; any shape works if all the pieces are the same approximate size. Pack the fillets into a sterilized jar.
Add equal parts distilled white vinegar and filtered water to a heavy-bottomed saucepan, enough to cover the fish. For very tart pickled fish, use 3 parts vinegar and 2 parts water.
Add flavoring ingredients and bring the solution to boil. Add 1/2 tablespoon of salt per cup of solution.
Boil the solution until the sugar and salt dissolve. Pour the boiling solution over the fillets; leave about 1/2 inch of space at the top of the jar.
Seal the jar with the metal lid and screw cover. Let the solution cool to room temperature and store up to a month in the refrigerator. Wait 24 hours before eating the fish.