- Preheat the oven to 180 °C (350 °F).
- Peel and cut the potatoes and celeriac into 1-cm (1/2-inch) thick slices.
- In a 25-cm (10-inch) cast iron skillet, brown the onion and garlic in the oil.
- Place the potatoes and celeriac in successive layers on the onion mixture. In a bowl, combine the cream, cheese, egg, thyme and nutmeg. Season with salt and pepper. Pour over the vegetables.
- Cover and bake for about 1 hour. Let rest for 10 minutes before serving.