Start the oven on 200 degree Celsius on electrical heat.
Peel the potatoes and the onion and grate them in to thin slices.
Put 1/3 of the potatoes in the bottom of the oven form, and spread ½ of the onion on top of the potatoes.
Open the tin with the anchovy fillets, and place ½ the anchovy fillets on the potatoes.
Sprinkle a thin layer of salt and white pepper. Repeat this process with another layer
Place the rest of the potatoes as a third layer.
Pour 2/3 of the anchovy fillets liquid (brine) evenly over the gratin.
Pour the cream evenly over the gratin and make sure it goes down in the gratin.
Pour the bread crumbs as an even layer on the gratin, and dot/spread with butter on top of the bread crumbs.
Place the gratin in the middle of the oven during 40-45 minutes.
Take out the gratin and serve it warm.
Tradition is to serve it with Swedish meatballs.