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1
Peel and julienne the potatoes to 3–4 mm. Rinse the julienne in cold water until the water runs clear, then pat thoroughly dry with a clean cloth. This removes surface starch that would thicken the cream prematurely and produce a gluey sauce.
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2
Melt the butter in a wide pan over a medium-low heat. Add the sliced onions and cook slowly, stirring occasionally, until they are soft, translucent, and beginning to colour at the edges. The onions are done when all sharpness has left them and they carry a gentle sweetness — this stage cannot be rushed.
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3
Butter a gratin dish. Lay half the potato julienne across the base in an even layer. Distribute the ansjovis fillets and all of the caramelised onion across the potatoes. Cover with the remaining potato julienne. Pour 250 ml of cream evenly over the surface. This is the critical layering stage: the ansjovis must be interiorly placed so that their spiced, saline character penetrates upward and downward through the cream as the dish bakes — too little fish coverage at this point and the dish lives as mere cream gratin; too much salt or the wrong fish and it dies entirely.
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4
Cover the dish tightly with foil and place in an oven preheated to 200 °C. Bake for 20 minutes. The cover traps steam that begins to cook the potato through before the cream reduces.
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5
Remove the foil. Stir the reserved tablespoon of ansjovis brine into the remaining 250 ml of cream, then pour this over the surface of the gratin. Return the dish to the oven uncovered. Bake until the top is deep golden-brown, the cream has been absorbed into a cohesive, savoury sauce, and a knife pressed into the centre meets no resistance from the potato. The surface should be visibly set and coloured, not pale or bubbling liquid.
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6
Remove from the oven and allow to rest for five minutes before serving. Taste before adding any salt — the ansjovis contribute significant salinity and additional seasoning is rarely required.