A perfect gratin is when the cream and milk become rich and thick like paint. François cooks it until the sauce around the potatoes would be good for a ‘painter’, he tells me laughing! This makes a generous amount but it is real comfort food that just demands seconds.
Preheat the oven to 240°C/475°F/Gas 9. Wash the potatoes in water to remove any excess starch, then shake dry. Slice into 4mm (⅙-inch) wafer-thin rounds — a mandolin is best if you have one, or you could use the slicer attachment on a food processor.