Preheat the oven to 325°F / 160°C. Butter a 7 × 11in / 18 × 28cm gratin dish.
Peel 1kg of Desirée potatoes and slice them ⅛in / 3mm thick. Pat dry with a kitchen towel.
Slowly heat 300ml of whole milk, 284ml of heavy cream, 1 halved garlic clove, and 2 thyme sprigs until almost boiling. Remove from the heat.
Layer half the potatoes in the dish, slightly overlapping and seasoning with salt and pepper. Pour over half the hot cream. Layer the rest neatly on top and pour over the remaining cream.
Bake for about 1 hour, until golden and tender. Let stand for 5 minutes before serving.
