Peel the potatoes and cut them lengthwise, French-fry style, into batons 1/2 inch thick. Set aside. Slice the onions into rounds about 1/4 inch thick and set these aside also.

Preheat an oven to 350° F. Butter a 2-quart baking dish, preferably one with a fitted lid. Arrange about one-third of the potatoes in a layer in the prepared dish. Scatter about half of the anchovy pieces over them and sprinkle with about half of the rosemary. Top with a layer of half the onions. Repeat using half of the remaining potatoes and all of the remaining anchovies, rosemary, and onion. Layer the remaining potatoes on top.

In a pitcher, stir together 3/4 Cup of the cream and all of the oil from the anchovy tin. Pour the mixture over the top of the potatoes. Cover the dish with its lid, or cover and seal with aluminum foil.

Place it in the oven and bake for 35 minutes. Remove from the oven, remove the lid or cover, and pour the remaining cream evenly over the potatoes. Sprinkle the top with the pepper. Cover again and bake until the potatoes are tender enough to break apart when pierced with the tines of a fork, about 15 minutes longer. Serve hot, spooned from the baking dish.