- 1½ pounds large Yukon Gold or russet potatoes, peeled and very thinly sliced
- 1½ pounds celeriac (celery root), peeled and very thinly sliced
- 2 cups whole milk
- 2 cups cold water
- 3 plump cloves garlic, peeled, halved, green germs removed, minced
- ¾ teaspoon fine sea salt
- 3 bay leaves, preferably fresh
- A bunch of fresh thyme, tied with cotton twine or secured in a wire mesh ball
- Freshly grated nutmeg
- Freshly ground black pepper
- 1 cup heavy cream
- 2 cups (about 8 ounces) freshly grated imported French or Swiss Gruyere cheese
- 1 tablespoon fresh thyme leaves
- A 2-quart gratin dish
- Preheat the oven to 375 degrees F.
- Place the potatoes and the celeriac in a large saucepan and cover with the milk and cold water. Add the garlic, salt, bay leaves, and bunch of thyme. Bring to a simmer over medium, high heat, stirring occasionally to prevent the vegetables from sticking to the bottom of the pan. Reduce the heat to medium and cook, stirring from time to time, until the vegetables are tender but not falling apart, about 10 minutes.
- Using a slotted spoon, transfer half of the vegetables to the gratin dish. Discard the milk and water mixture and the thyme. Season to taste with nutmeg and pepper, and cover with half of the cream and half of the cheese. Cover with a final layer of vegetables, cream, and cheese. Season again with nutmeg and pepper. Sprinkle with the thyme leaves.
- Place in the center of the oven and bake until crisp and golden on top, 1 to 1½ hours. Serve immediately.