Step 1

Peel the onions and slice into fine half-moons, and melt 3 tablespoons of the butter in a large frying pan; I use a non-stick one of 28cm / 11in diameter. Add the onions to the pan, sprinkle with the ½ teaspoon of sea salt flakes (or ¼ teaspoon of fine sea salt) and stir for a few minutes over medium-high heat. Once they start to lose their raw look, turn the heat down to low and cook very gently for 15–20 minutes. Keep an eye on them, and stir regularly: they should be soft and pale gold. If you think they are beginning to catch, you can sprinkle a very little water into the pan, or simply add more butter. Once they’re ready, use a bendy spatula to scrape them and their buttery juices into a bowl to stop them burning in the hot pan.

Step 2

Meanwhile, heat the oven to 200℃/180℃ Fan/400°F. Butter an oven dish of about 2½ litres / 2½ quarts capacity; I use one (an old friend that you may have seen in a few of my books) of 34 x 20 x 6cm / 13 x 8 x 2½in.

Step 3

Peel the potatoes, cut them into narrow strips rather like French fries, then mound them into three roughly equal piles.

Step 4

Put a third of the potatoes into the bottom of the buttered dish. Season with 1 teaspoon of sea salt flakes (or ½ teaspoon of fine sea salt) and a good grinding of pepper.

Step 5

Spread half the soft onions over the potato strips, followed by one tin of ansjovis; it is up to you whether you add the liquid in the tin or not (see recipe intro).

Step 6

And now layer up again: so, the second third of potatoes, salt and pepper as before, followed by the rest of the onions and the second tin of ansjovis.

Step 7

Top with the final third of potatoes, then mix together the milk and cream in a jug and pour it over, although it won’t cover the potatoes completely. Push any that are poking up down into the creamy milk, just so that they’re lightly covered, but don’t worry if they pop up again.

Step 8

Sprinkle over the breadcrumbs if using (I confess I forgot to add them for the photo) and dot over the remaining tablespoon of butter, then bake in the oven for 1 hour, making sure you check at 45 minutes; push a fork in, and if it meets with no resistance, the potatoes are cooked. They’ll certainly be golden on top, but it’s essential they’re soft underneath.

Step 9

The gratin will stand for a while until needed, and tastes just as good, if not better, warm rather than hot. Leftovers are easily reheated for repeated rapture. For many Swedes – and those of us who are not Swedish – it is these reheated leftovers that are the real Christmas treat.