1. Prepare the oven and dish: Preheat your oven to 160°C. Use the butter to grease a large baking dish thoroughly.
2. Season the cream: In a large bowl, whisk together the double cream, crushed garlic, and grated nutmeg. Season generously with salt and pepper.
3. Slice the vegetables: Using a sharp knife or, for best results, a mandolin, finely slice the peeled celeriac and potatoes into thin rounds.
4. Layer the gratin: Dip the vegetable slices into the cream mixture to ensure they are fully coated. Arrange them in the baking dish in even, alternating layers.
5. Press and pour: Press down firmly on the layers with your hands or a spatula, then pour any remaining cream mixture over the top.
6. Slow bake: Place in the oven and bake for 1 ¼ to 1 ½ hours. Occasionally press down on the vegetables with a spatula during cooking to keep them submerged in the cream.
7. Crisp and finish: The dish is ready when the vegetables are tender, and the top is bubbling and golden. For a deeper crust, turn the oven up to 190–200°C for the final 5 minutes of cooking.

Serve: Let it sit for a few minutes before serving to allow the cream to set slightly.