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1
Peel the potatoes and slice them to 4–5 mm on a mandoline or by hand — thicker than for a gratin dauphinois, thin enough to cook through without a raw centre. Keep the slices dry; do not rinse them, as surface starch aids the delicate exterior.
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2
Place the duck fat in a wide, heavy-based pan over a moderate flame. Heat until the fat shimmers and moves freely across the surface but produces no smoke — 150–160 °C. This temperature range is where the dish lives or dies: too high and the exterior hardens before the interior softens; too low and the potatoes stew rather than confite. Adjust the flame to hold this window throughout cooking.
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3
Add the potato slices in a single layer, working in batches if the pan cannot hold them without overlap. Each slice must have full contact with the fat; overlapping causes steam and produces an uneven, sodden result. Cook gently for 25–30 minutes, turning occasionally with a palette knife. The sign that the batch is ready: each slice is yielding and creamy when pressed at its thickest point, and the underside carries a pale, even gold.
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4
In the final 5 minutes of cooking, scatter the thinly sliced garlic and the parsley evenly over the potatoes. The garlic is done when it is golden and fragrant — if it darkens to brown it will turn bitter and the dish is compromised. The parsley will wilt and colour slightly in the fat.
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5
Season immediately with fleur de sel and cracked black pepper. Serve directly from the pan. Pommes Sarladaises lose cohesion and perfume rapidly once lifted from the fat; the pan is the correct vessel.