Combine wine, vinegar, sugar, pepper and coriander seeds in a pot and bring just to a boiling point (but do not boil). Remove from heat and allow to cool. Add herring with their juices and let marinate for at least 12 hours. If desired, cut herring into bite-sized pieces before marinating.
Peel onions and cut into rings or cubes.
Arrange herring on plates and sprinkle with onions. Serve immediately.