You will need a gratin dish and some foil. Layer the potatoes and the onions with the anchovies in the dish, peppering as you go. Finish with potatoes on the top layer.
Pour over half the cream and a little of the anchovy oil. Dot with a little butter if you like. Cover with the foil and cook in a hot oven for 20 minutes. Then pour in the rest of the cream and turn the heat down low. The potatoes will take another hour to cook. Remove the foil for the last 10 minutes.
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