Janssons Frestelse

Ingredients

• 1kg floury potatoes such as Maris Piper or King Edward
• 3 large yellow onions
• 125g tin Swedish ansjovis (cured sprat fillets), plus 3 tbsp of their brine
• 400ml double cream
• 100ml whole milk
• 40g butter, plus more for the dish
• 3 tbsp dried breadcrumbs
• White pepper
• Salt, only if needed

Before you cook

• Butter a 2 litre gratin dish generously, right up the sides.
• Peel the potatoes and cut them into matchsticks about 5mm thick and 6cm long, using a knife rather than a grater so they hold their shape.
• Drop the potato sticks into a bowl of cold water and leave them there until needed.
• Peel the onions, halve them and slice them thinly.
• Drain the ansjovis fillets over a bowl and keep 3 tablespoons of the spiced brine.
• Cut each fillet in half lengthways if they are large.

Steps

1. Heat your oven to 200C fan.
2. Melt half the butter in a wide frying pan over medium-low heat.
3. Add the sliced onions with a pinch of salt and cook them for 12 minutes, stirring often, until soft and translucent but not browned.
4. Drain the potato matchsticks and dry them thoroughly in a clean tea towel, because wet potatoes make a watery gratin.
5. Spread a third of the potatoes over the base of the buttered dish and season with white pepper.
6. Scatter half the onions and half the sprat fillets over the potatoes.
7. Repeat with another third of the potatoes, then the rest of the onions and sprats.
8. Finish with the last of the potatoes, pressing the surface flat with your hands.
9. Whisk the cream, milk and reserved sprat brine together and pour it evenly over the dish.
10. Scatter the breadcrumbs over the top and dot with the remaining butter.
11. Bake for 45 minutes, then check that a knife goes through the centre without resistance.
12. If the potatoes are still firm, cover the dish loosely with foil and give it another 10 to 15 minutes.
13. Rest the gratin for 10 minutes before serving so the cream sets around the potatoes.
14. Taste before adding salt, since the sprats usually carry all the seasoning the dish needs.