In a glass measure, combine the vinegar and sugar until the sugar is completely dissolved. Add the aquavit, allspice and juniper berries.
Drain the jarred herring in a colander and rinse in cold water. Remove the herring pieces, pat dry with paper towels and cut into uniform bite-sized pieces. Place half of the herring in a single layer in a glass bowl. Top with half of the onions and dill, repeat with the remaining ingredients and pour the marinade over the fish.
Cover and refrigerate for at least 4 to 6 hours. To serve, drain the excess liquid and discard the berries. Garnish with additional fresh dill if desired.
Notes
ABOUT THE AQUAVIT, ALLSPICE & JUNIPER: Aquavit is a Scandinavian spirit flavored with caraway or dill, often with added spices like fennel, anise, or citrus peel. It’s usually served chilled and sipped alongside Nordic dishes like smoked fish or pickled herring. We add a tiny splash to this recipe to bring out the flavors of the whole allspice and juniper berries. If you don't have these ingredients on hand, they can be omitted without sacrificing too much of the flavor.