Heat half the butter and stir in the anchovies till they melt (save the oil) Drain the anchovies, reserving their oil.
Add the onions and gently cook for 20 mins (make sure they don’t brown).
Line a baking tray with the rest of the butter and arrange half the potatoes, then pour over the onions: Add the remaining potatoes and half the cream and bake for 30 minutes.
Add the remaining cream and bake for another 25 minutes, until golden!