This is a very popular dish in Swedish holiday cooking. It's always present at christmas (and it's perfect with the meatballs and the ham) but it's common at Easter too, or just as a late-night snack.
It's very salty but still sweet - Swedish anchovies are cured in a sweet brine and not very fishy. You really DO need those, there's no substitute, so I hope you have a Swedish grocery source somewhere near you.
4-5 medium potatoes, roughly grated
1 small can of Swedish flat anchovies (125 g)
1 large onion, thinly sliced
300 ml of heavy cream
Preheat the oven to 225°C. Mix the potatoes with the onions, and add a little bit of salt and pepper. Put half of the mix in a buttered oven proof dish. Top with all of the anchovies. Add the rest of the potatoes and onions. Drizzle over half of the cream, and put in the oven. After about twenty minutes, add the rest of the cream. Bake until it has some color - about twenty more minutes, so 40 minutes in total.
Recipe in Swedish:
Home Unlabelled Holiday Cooking: Jansson's Temptation