This is a gratin-style dish that everyone in Sweden knows well. Never make the mistake of using anchovies, as many English-language recipes suggest. The Swedish word ‘ansjovis’ actually means sprats, not anchovies. Whoever originally translated it that way condemned many a poor person to a very salty dish!
Preheat the oven to around 200C/400F/Gas 6.
Peel the potatoes and chop them into small 0.5cm/¼in piece sticks - a bit thinner than French fries.
Melt the butter in a saucepan, add the onion and cook until soft. Take care not to burn the onion - it should be cooked, but not turning brown.
Add the potatoes and cook for a few minutes.
Layer half of the onion and potato mixture in the bottom of an ovenproof dish, then top with 10 sprat fillets evenly across. Season with salt and pepper.
Add another layer of onion and potato, then another 10 sprats on top. Pour the remaining sprat juice over the dish.
Pour two-thirds of the milk and cream over the dish, then sprinkle the breadcrumbs on top. Season again.
Bake for about 45 minutes, or until the potatoes are cooked. Halfway through the cooking time, add the rest of the milk and cream. If the dish is looking dry, you can add more milk - the aim is to get a creamy consistency.
Serve as part of a traditional Swedish smörgåsbord.