The keys to a successful Jansson's Temptation are two-fold: the potatoes must be cut into thin matchsticks (not slices) and the correct fish must be used. Swedish 'sardeller' (anchovies) are actually cured fillets of sprats with an intensely salty brine – not the classic Mediterranean anchovies.
This dish should be golden-brown after baking, creamy inside, and pleasantly salty. It is the perfect contrast to sweet desserts and rich meat dishes.
Preheat the oven to 200 °C (390 °F).
Peel the potatoes and cut them into thin, long strips (julienne) – they should look like thick matchsticks. This is key for the right texture.
Peel the onion and slice it into thin half-moons.
Drain the sprat fillets and save the brine. Cut the fillets into smaller pieces if they are too long.
Thoroughly grease a baking dish (ideally ceramic) with butter.
Spread half of the potato strips on the bottom of the dish. Evenly spread the sliced onion on top and lightly season with black pepper (Do not add salt, the brine will take care of that!).
Arrange the sprat fillets on top and cover with the remaining potato strips. Season the top again with black pepper.
In a small bowl, mix the cream with the reserved sprat brine. Pour this mixture evenly over the potatoes. The liquid should almost cover the potatoes.
Sprinkle with breadcrumbs and dot the top with small pieces of butter.
Bake in the preheated oven at 200 °C (390 °F) for 50 – 60 minutes, or until the potatoes are tender (test with a skewer) and the surface is golden brown and crunchy.
If the surface browns too quickly, cover the dish with aluminum foil.
Serve hot, straight from the baking dish.