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1
Place the unpeeled potatoes in cold salted water and bring to the boil. Cook until a knife meets slight resistance at the centre — they will finish cooking in the pan. Drain, cool completely, peel, and slice 5–6 mm thick. Refrigerating overnight sharpens the slices and promotes crispness.
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2
Melt the butter in a heavy pan over medium-low heat and add the onion half-moons. Cook slowly for 15–20 minutes, stirring occasionally. The onions are ready when they are deeply golden, collapsed, and sweet throughout — not merely softened and translucent.
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3
Heat the clarified butter in a wide sauté pan over medium-high heat until it hazes. Arrange the potato slices in a single layer — work in batches if the pan cannot accommodate them without overlap. Sauté for 8–10 minutes, turning occasionally, until both surfaces are golden and crisp. This is where the dish lives or dies: crowding the pan at this stage converts dry heat to steam and the potatoes will never crisp; a single layer and sufficient heat are without compromise.
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4
Fold in the caramelised onions, tossing gently so the onion distributes evenly without breaking the potato slices. The mixture should smell of browned butter and sweet onion throughout.
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5
Deglaze the pan with the red wine vinegar. The acid lifts the caramelised sugars from the pan floor; the sound of the liquid hitting the hot pan and then subsiding signals the deglaze is complete. Taste: the dish should carry a barely perceptible tang beneath the butter richness.
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6
Shower with chopped parsley and serve immediately. The contrast between crisp potato and yielding onion fades within minutes of leaving the pan.