Rinse the Salted Herring Fillets briefly under cold water to remove surface salt, then soak them in cold Water for 2–4 hours (change the water once) if they're very salty; drain and pat dry.
Cut the fillets into bite-sized pieces or leave whole according to preference and set aside.
In a saucepan combine 1 part Spirit Vinegar (12%) with 1 part Water (adjust volume for the amount of fish), add Granulated Sugar to taste and heat gently, stirring until the sugar dissolves.
Add the spice aromatics—Allspice Berries, White Peppercorns and Bay Leaves—to the simmering brine and let them steep for 3–5 minutes to release their flavors; remove from heat.
While the brine is warming, thinly slice the Red Onion and peel and cut the Carrot into thin rounds or matchsticks.
Arrange the herring pieces in a clean jar or shallow container in layers with the sliced onion and carrot, distributing the spices from the brine among the layers.
Pour the hot pickling liquid over the herring and vegetables so everything is fully submerged; let the jar cool to room temperature, then cover and refrigerate.
Allow the pickled herring to rest in the refrigerator at least 24–48 hours (better for 2–3 days) so the flavors meld. Serve chilled with crusty bread, boiled potatoes, or as part of a smorgasbord.
