Preheat the oven to 190°C fan.
Begin with the onion. Heat a medium saucepan to medium/high and add half the butter and the sunflower oil. Add the onion and cook on a medium to low heat for 5-10 minutes or until very soft and lightly golden.
If you have a mandolin, cut the potatoes into julienne, otherwise slice the potatoes then cut into matchsticks. Do not rinse them.
If you are using the Swedish anchovis, drain away half of the liquid from the tin and reserve the rest for to mix with the cream. This liquor acts as seasoning for the dish. If you are using standard anchovies in olive oil, drain the olive oil then place the fillets in a bowl with the other ingredients and add 30mls hot (not boiling) water and leave for 20 minutes.
Chop the anchovies. Grease an ovenproof dish with a little butter. Add a layer of potato, followed by some of the sautéd onion then a scattering of anchovies. Season with white pepper and a very light sprinkling of salt. Repeat with, potato, onion and anchovy. Finish with a layer of potato.
Pour the cream with the milk, anchovy liquor or water into a saucepan and heat to boiling point, then pour over the potatoes. Sprinkle with the breadcrumbs and dot with the remaining butter. Cover with foil and place on a low oven shelf. bake for 20 minutes, then remove the foil and reduce the heat to 180°C fan. Continue to bake for 40-45 minutes or until the potatoes are golden and very soft. Serve as part of a Smörgåsbord or with a green salad and crusty bread.