Preheat the oven on 200 degrees Celsius.
Peel and grate the potatoes in slices.
Peel and cut the onion in small slices.
Cover the bottom and edges of an oven form with butter.
Spread 1/3 of the grated potatoes, and ½ of the onion in the oven form. On top of the potatoes place evenly ½ of the anchovy fillets. Make sure to keep the brine you will need it later on.
Sprinkle a little salt and white pepper on top of the fish fillets.
Make another layer of potatoes, onion, anchovy fillets, salt and pepper.
Place the rest of the potatoes as a last layer.
Pour 2/3 of the fish brine and all of the cooking cream over the gratin.
Sprinkle the bread crumbles evenly on the gratin, and place the butter in small pieces over the crumbles.
Place the gratin in the middle of the oven for 20 minutes. Pour the rest of the brine on the edges of the gratin. Keep it in the oven for 20 more minutes.
Take it out and serve it warm.