Place a layer of sliced potatoes onto the bottom of a large baking dish. Top the potatoes with a layer of sliced onions and then a layer of anchovy fillets. Repeat layers up to the top of the dish, ending with a layer of potatoes on top. Drizzle about 2/3 of the cream over the dish, and the reserved liquid from the anchovies. Sprinkle the breadcrumbs over the top and dot with pieces of butter.
Bake for 30 minutes on the middle rack of the oven. Add the remaining cream and continue baking for another 15 minutes or until the potatoes are tender.