Preheat oven to 220 C / Gas 7. Butter a 20x30cm 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 double cream. Mix breadcrumbs together with the melted butter, then sprinkle over of the potatoes.
Place onto a baking tray to catch any cream that 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.