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1
Place the potatoes, unpeeled, in a large pan of cold salted water. Bring to the boil and cook until just tender — a skewer should meet slight resistance at the centre. Drain, allow to cool enough to handle, peel, and slice to 1 cm thickness.
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2
Set a wide frying pan over medium heat. Add the lardons and render until golden and crisp, stirring occasionally. Remove with a slotted spoon and set aside, leaving the rendered fat in the pan.
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3
In the same fat, cook the sliced onions over medium-low heat for 10–12 minutes, stirring regularly, until soft and evenly golden. They should carry no raw sharpness when tasted.
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4
Combine the sliced potatoes, lardons, and onions in a gratin dish. Season with black pepper only — the lardons and Reblochon carry sufficient salt. Deglaze the frying pan with the white wine, scraping up any residue, and pour the liquid over the assembled potato mixture. This is where the dish lives or dies: without the wine deglaze, there is no acidity to cut the richness of the cheese and lardons, and the finished gratin is flat and cloying.
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5
Cut the Reblochon in half horizontally to yield two equal discs. Lay both discs cut-side down on top of the potato mixture so that the cut surfaces face the potatoes and the natural rind faces upward.
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6
Bake in an oven set to 200 °C for 20–25 minutes. The gratin is ready when the Reblochon is visibly bubbling and oozing at its edges and the rind has turned a wrinkled golden-brown. The cheese should be molten beneath the rind yet still hold its shape as a cap — it must not have liquefied entirely and pooled to the bottom of the dish.
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7
Serve immediately, cutting through the cheese cap so that each portion releases molten Reblochon onto the potatoes below. Accompany with a green salad dressed with walnut oil. The cheese sets as it cools; delay is not an option.