- 2 pounds baking potatoes, peeled and cut crosswise into 1/4-inch slices
- 5 cups whole milk
- 2 cloves garlic, very lightly crushed
- 1 1/2 cups heavy cream
- 1 1/2 cups crème fraiche (or sour cream)
In a large saucepan over medium heat, bring the potatoes, milk, and garlic to a gentle simmer until just tender, about 15 minutes.
Position a rack in the center of the oven and preheat to 350 F.
Transfer the potatoes to a medium bowl with a slotted spoon, taking care not to break the slices. Drain the milk from the pan and reserve for another use. Add the heavy cream, crème fraiche or sour cream, salt, pepper, and nutmeg to the saucepan and whisk to combine. Gently return the potatoes to the pan.
Heat the potato-cream mixture to barely a simmer (a few bubbles are fine but no more) for 10 to 15 minutes until the potatoes are very tender but not falling apart.
Butter a large baking dish. Transfer the potatoes to the baking dish with a slotted spoon, layering them without breaking them up too much.
Cover with the cream from the pan.
Sprinkle the Gruyere cheese over the potatoes (if using).
Bake in the oven until the potatoes are golden brown on top and the cream is mostly absorbed, about 50 minutes.