1. Preheat the oven to 400°. Place the potato slices in a bowl of cold water to prevent them from discoloring.
2. Heat 2 tablespoons of the butter in a small frying pan; when the foam subsides, add the sliced onions and cook them over moderate heat, stirring frequently, for 5 to 8 minutes, or until the onions are soft and transparent but not brown. Set them aside.
3. Choose a 1- to l˝-quart baking dish attractive enough to bring to the table and, with a pastry brush or paper towels, spread with the remaining ˝ tablespoon of butter. Drain the potatoes and pat them dry with paper towels.
4. 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 layer of potatoes with bread crumbs and dot with the bits of butter.
5. Pour in the cream, bring to a boil on top of the stove, then bake for 1 hour in the center of the oven, until the potatoes are tender when pierced with the tip of a sharp knife. Serve from the baking dish.
