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1
Place the cut batons in a large bowl of cold water with the white vinegar. Soak for 30 minutes. Drain, then dry thoroughly in a clean cloth or on paper — no surface moisture should remain.
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2
Heat the oil or tallow in a deep, heavy pot to 140–150 °C. Lower the batons in batches; do not crowd the vessel. Fry for 6–8 minutes. This is where the dish lives or dies: the frites must emerge completely pale — no colour whatsoever — yet cooked through to a soft, tender interior. Colour at this stage means the exterior will burn before the interior is fully steamed through in the second cook.
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3
Lift the blanched frites onto a wire rack and cool completely. For an ultra-crisp, rough-textured exterior, transfer the rack to a freezer for 20 minutes — the ice crystals that form on the surface shatter during the second cook.
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4
Raise the oil temperature to 180–190 °C. Return the frites in batches and fry for 2–3 minutes. The rapid surface dehydration and Maillard reaction produce the characteristic glass-crisp shell. Remove when the frites are deep golden in colour.
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5
Transfer immediately to a wire rack or paper-lined bowl. Season with fine salt the instant they leave the oil — the residual oil on the surface is the only window in which the crystals will adhere. Serve within 5 minutes. A correct frite is audibly crisp when bitten, yielding to a soft, potato-flavoured interior.