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Preheat the oven to 200°C, fan 180°C, gas 6. Cut the potatoes into ½ cm slices, then into matchsticks (you can use a mandoline attachment if you have one). Put in a pan or bowl of cold water as you prepare them, to prevent browning and to get rid of excess starch.
Heat 15g of the butter in a large frying pan and fry the onions on a medium heat, stirring occasionally, until they are golden and transparent – 20-30 minutes.
Butter a large ovenproof dish, about 2.5 litres. Put in one layer of potato – use a third – then a layer of half the cooked onion. Add 5 of the anchovies, spreading them out evenly, then a few small cubes of butter. Repeat these layers, finishing with a final layer of potato sticks. Pour over the cream, press down, then scatter with the breadcrumbs. Putting some more butter on top will make the crumbs even crisper. Season with black pepper.
Bake for 40-55 minutes, or a little bit longer if using the hot oven of an Aga, until the potatoes are tender (the timing will depend on just how thinly your potatoes are cut). Press the potatoes down with the back of a spoon after 20 minutes to submerge all the potato in the cream. The dish is ready when the potatoes are soft when pierced with a knife, and the top is nicely brown and crisp.
Recipe from Kirstie’s Real Kitchen (Hodder & Stoughton, £25).