Janssons frestelse — Swedish potato and sprat gratin

Preheat the oven to 200°C, conventional heat. Have an ovenproof dish about 30 × 20 cm, at least 6 cm deep and about 3 l in capacity ready; grease it with 5 g butter. Drain the sprats, reserve 50 ml brine and weigh out 150 g fillets, then break them into shorter pieces. Cut the peeled potatoes into thin strips about 3 mm thick and the onion into thin half-moons. Do not rinse the cut potatoes, to retain the starch that thickens the sauce.

Melt 20 g butter in a frying pan over medium-low heat. Add the onion and cook for 8–10 minutes, stirring, until soft and translucent but not brown. Keep all the onion together with the butter from the pan.

Pour the cream and the measured 50 ml sprat brine into a saucepan, add the white pepper and stir. Heat gently until hot and starting to steam, without prolonged boiling. Taste: the brine and fish are already salty, so add only as much of the prepared 1 g salt as is actually needed, or leave it out entirely. Do not salt the individual layers separately.

Spread one third of the potatoes over the bottom, followed by half the onion and half the sprats. Repeat these layers, then top with the remaining potatoes. Pour the hot cream evenly over the dish and gently level with a spatula. The liquid should fill the gaps between the strips but not completely cover the top layer; leave some space below the rim for bubbling too. Sprinkle evenly with breadcrumbs and scatter small pieces of the remaining 35 g butter over them.

Place the dish in the lower part of the oven and bake for about 75–90 minutes. Check the top after about 35 minutes: if it is already golden, cover loosely with aluminum foil for the rest of the baking time. Near the end, slide a thin knife into the very center of the gratin all the way to the bottom — the potatoes should be completely tender, without hard, crunchy pieces, and the sauce thickened between the layers. If needed, bake for about 15 minutes longer under the foil. If the top is still pale, remove the foil for the last few minutes.

After removing the gratin from the oven, let it stand for about 10 minutes without covering. The sauce will thicken slightly and the topping will stay crisp. Divide into four servings and lift out with a wide spatula, including all the creamy sauce from the bottom; the center should be soft and moist, not dry or liquid like soup.