Boil the potatoes in water until soft, then cool and cut into slices.
Heat 1 tablespoon oil in a large skillet. Pour the onion into the pan and fry over low heat, stirring occasionally, for about 5 minutes, until soft. Remove the pan from the stove and set it aside.
Grease slices of bread with the rest of the butter on one side. Put 1 leaf of lettuce on slices of bread and cut the bread and leaf in half.
Place each slice of bread on a serving plate and place 4-6 potato slices on top.
Put 4-6 slices of herring in the center of each slice of bread, and put fried onions on top, 2 tablespoons each.
Sprinkle chopped chives over each sandwich and serve immediately.