Preheat oven to 425 degrees F (220 degrees C). Butter a 9×13 inch baking dish.
Spread onions evenly onto the bottom of the prepared baking dish. Evenly distribute anchovies on top of the onions. Place potatoes on top of the anchovies, then pour in heavy cream. Mix bread crumbs together with the melted butter, then sprinkle overtop of the potatoes.
Place onto a baking sheet to catch any cream which bubbles over. Bake in preheated oven until the potatoes have softened, the cream has reduced, and the dish has turned deep golden brown, about 45 minutes.
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