Preheat oven to 400�F. Place potato slices in a bowl of cold water to prevent them from discoloring. Heat 2 tb of butter in a small frying pan and, when the foam subsides, add the sliced onions, cooking until they are soft and transluscent but not brown. Set aside. Choose a 1 to 1 1/2 quart baking dish attractive enough to bring to the table and using a pastry brush or paper towels, spread with the remaining 1/2 tablespoon of butter. Drain potatoes and pat dry. Arrange alternate layers of potatoes, herring and onions in the baking dish, ending with a layer of potatoes. Season each layer lightly with pepper, sprinkle the top with breadcrumbs and dot with bits of butter. Pour in the cream, bring to a boil on top of the stove and then bake for 1 hour in center of oven until potatoes are tender when pierced with the tip of a sharp knife or skewer. Serve directly from baking dish.