I first ate this immensely popular Swedish dish at the house of Swedish friends and found it quite wonderful. The saltiness of the anchovies gives flavour to the potatoes and the cream adds a delicate note. Do not omit or reduce the amount of anchovies, they are essential to the dish and not overwhelming.
Heat the oven to 200ºC, 400ºF, gas 6 and butter a gratin dish with half the butter. Arrange half of the potatoes in the dish and cover with the onion and anchovies. Top with the remaining potatoes. Smooth the top and pour over the cream. Dot with the rest of the butter. Bake in the oven for 50-60 minutes. If the top is browning too quickly, cover with foil. Leave the dish for a few minutes before serving because it will be very hot.