If using fresh herring fillets, clean and debone them. If using pickled herring fillets, rinse them under cold water to remove excess salt or brine.
In a saucepan, combine the white vinegar, water, sliced onion, peppercorns, allspice berries, bay leaf, sugar, and a pinch of salt. Bring the mixture to a boil, then reduce the heat and let it simmer for about 10 minutes.
Remove the saucepan from the heat and let the marinade cool completely.
Once cooled, place the herring fillets in a shallow dish and pour the marinade over them, making sure they are fully submerged. Cover the dish and refrigerate for at least 2 hours or overnight to allow the flavors to develop.
After marinating, remove the herring fillets from the marinade and pat them dry with paper towels.
In a large frying pan, heat enough vegetable oil to cover the bottom of the pan. If desired, you can lightly coat the herring fillets in flour before frying for a crispy texture.
Fry the herring fillets in batches over medium-high heat until golden brown on both sides. This should take about 3-4 minutes per side.
Once fried, transfer the Brathering to a plate lined with paper towels to absorb any excess oil.
Serve the Brathering warm with your favorite accompaniments, such as boiled potatoes and a fresh salad.
