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 1.5 quart baking dish attractive enough to bring to the table and using a pastry brush or paper towels, spread with the remaining half 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.