Method

• 1.
  Preheat oven to 190°C/170°C fan-forced.
• 2.
  Melt half the butter in a large frypan over medium heat. Cook onion and garlic, stirring occasionally, for 6 minutes, or until softened. Brush a little of the onion butter around a 2L-capacity baking dish. Season with freshly ground black pepper. Pour cream into a jug, and add remaining onion mixture.
• 3.
  Melt remaining butter in same pan over medium-high heat. Add leek and thyme and season. Cook, covered, stirring occasionally, for 10 minutes, or until softened. Add wine and cook, uncovered, for 5 minutes.
• 4.
  Peel and slice potatoes 2mm thick using a mandoline (do not wash or soak sliced potatoes in water or you will remove their starch). Layer half the potatoes, cream mixture, parmesan and cheddar alternately to half-fill baking dish. Top with leek mixture, then repeat with remaining potato, cream mixture, parmesan and cheddar. Season and grate over a little nutmeg. Cover with foil and bake for 1 hour 30 minutes, or until potatoes are tender.
• 5.
  Increase oven to 195°C/185°C fan-forced. Top potatoes with mozzarella. Bake for 15 minutes, or until golden. Sprinkle with extra thyme and serve.