Jansson's temptation

Step 1: Heat the oven to 400°F (350°F fan). Rub a 20 × 30 cm baking dish with a little of ⅓ oz butter.

Step 2: Melt half of ⅔ oz butter in a frying pan over a medium-low heat and cook 2 onions for 8–10 minutes until soft and golden at the edges.

Step 3: Rinse 2 ¼ lb firm potatoes in cold water and pat dry. Spread a third of them in the dish, then half the onions and half of 2 ¼ oz Swedish-style spiced sprat fillets torn into pieces. Repeat, and finish with a layer of potato.

Step 4: Mix 1 ¼ cups double cream, ½ cup whole milk, 2 tablespoons of the sprat brine and ½ tsp ground white pepper. Pour it over the potatoes; it should come about three-quarters of the way up. Taste a drop: it should be well seasoned, but add salt only if needed, as the sprats are salty.

Step 5: Scatter 2 tbsp dried breadcrumbs over the top if using and dot with the rest of ⅓ oz butter.

Step 6: Bake for 50–55 minutes until the top is golden, the cream has thickened and bubbles at the edges, and a knife meets no resistance in the middle. Cover loosely with foil if it browns too quickly.

Step 7: Rest for 10 minutes before serving so the sauce settles. It can be assembled a few hours ahead and kept in the fridge, then baked; leftovers keep 2 days chilled and must be reheated until piping hot.