Clarify butter by melting it in a saucepan or microwave, then set aside for a few minutes until milk solids settle on the base. Pour off the golden fat carefully, avoiding milk solids. You should get around ½ cup clarified butter (125 ml).
Peel and finely slice potatoes to 1.5mm thickness using a mandolin. Brush a 24cm cast-iron skillet with 2 tablespoons of clarified butter.
Build 3 layers of thinly sliced potatoes in a spiral pattern, brushing with clarified butter and seasoning as you go.
Bake for 70 minutes at 200°C / 400°F (180°C fan) covered with parchment paper cartouche, then 30 minutes uncovered.
Rest for 10 minutes, flip, and serve.
