Rinse the herring fillets under cold water and drain for 2 minutes. Place them in a large bowl, cover with cold water, and refrigerate for 24 hours, changing the water 2 times.
Make the brew: combine the spice mixture, salt, sugar, and a few quartered onions in a large saucepan. Add the water, orange juice, white balsamic vinegar, and herb vinegar. Bring to a boil over high heat.
Once boiling, turn off the heat and let the brew cool completely to room temperature.
Slice the remaining onions into thin slices. Strain the cooled brew through a sieve into a clean saucepan, pressing gently with a spoon.
Remove the herring fillets from the soaking water and rinse under cold water. In the saucepan with the strained brew, layer the onion slices and herring fillets alternately, ensuring the liquid covers everything.
Cover the saucepan and refrigerate for 2 days before serving.