Wash the herring fillets well under cold running water and place them in a glass or crockery bowl. Cover with cold water and refrigerate overnight (about 10-12 hours). Rinse herrings again and place them in a glass or enameled casserole or baking pan. Cover with sliced onions, making layers if necessary. Now make the marinade. In a saucepan combine vinegar, water, sugar, pickling spice, and bay leaves. Bring to a boil and reduce heat to a simmer. Cook for 3 to 5 minutes until sugar is dissolved. Cool slightly and pour over the herring. Cover and place in a cool spot or refrigerate and marinate for 2 to 3 days before serving. Remove the herring fillets from the marinade and slice into bite-size pieces. Place in a serving bowl, mix with marinade and onions, and serve.