1. Grind the allspice berries and black peppercorns in a mortar and pestle or spice grinder until coarsely broken. Set aside.

2. Peel the red onion and slice it thinly. Peel the yellow onion and cut it into bite-sized wedges.

3. In a large non-reactive bowl, combine the pickled herring, white wine vinegar, sugar, salt, ground spices, and sliced onions. Mix gently to avoid breaking the fish.

4. Add the bay leaves and whole cloves, then stir carefully to distribute all ingredients evenly.

5. Cover the bowl tightly with plastic wrap and refrigerate for at least 2 hours, preferably overnight, to allow the flavors to fully develop.

6. Before serving, transfer to a serving dish and garnish with additional bay leaves and cloves if desired. Serve well chilled.