Soak overnight in water or milk to cover:

Drain and split them into 2 fillets. Remove and discard the skin and bones. Cut the fillets into 1-inch-wide pieces. Preheat the oven to 375°F. Butter a 9-inch square baking dish.

Very thinly slice:

Arrange the potatoes, onions, and herring in alternating layers in the prepared baking dish, beginning and ending with potatoes. Cover the top with:

Bake until the potatoes are tender, about 45 minutes.