Steps
- Preheat the oven to 350°F / 180°C. Grease a shallow 14 × 9½in / 35 × 24cm baking dish.
- Melt 50g of butter in a skillet over medium heat. Cook 2 large halved sliced onions for 10 minutes; season and add 1 teaspoon of superfine sugar, then cook for 5-10 minutes, until golden.
- Heat 500ml of milk with ¼ teaspoon of grated nutmeg, 2 finely chopped garlic cloves, and the chopped leaves of 5 thyme sprigs. Season, bring to a boil, stir in 300ml of heavy cream, and remove from the heat.
- Peel and thinly slice 1 celery root and 1.25kg of potatoes. Layer the celery root, half the cream mixture, onions, and potatoes in the dish, arranging even potato slices on top.
- Pour over the remaining cream mixture and scatter with 50g of finely grated Parmesan. Bake for 1½ hours, until the layers are tender, then rest for 10 minutes.