Drain the herring and discard the liquid. Remove and discard the onion and spices.
Cut the lemon and orange into halves. Cut half of the lemon and half of the orange into the thinnest slices possible. Combine the slices with the herring. Squeeze the juice from the unsliced halves over the herring.
Add the vinegar, onion and sour cream and stir until blended. Let stand overnight in the refrigerator. Serve with dark bread and butter.