Preheat oven to 375 F degrees. Grease a 9x13” baking dish with butter or cooking spray and set aside.
In a large saucepan or stock pot, bring the milk and heavy cream to a boil. Reduce the heat and add the sliced potatoes, roasted garlic, thyme, salt, pepper and nutmeg. Simmer for 10 to 15 minutes, until the potatoes are tender.
Use a large spoon to transfer ⅓ of the potatoes into the prepared baking dish then sprinkle ⅓ of the cheese on top. Add another ⅓ of the potatoes and another ⅓ of cheese. For the final layer of potatoes pour over the cream mixture.
Bake for 20 minutes covered. Remove the foil and sprinkle on the remaining cheese. Bake uncovered for 10 minutes the top should brown slightly and the cream thickens. Cool for 10 minutes before serving.
Peel and slice the potatoes while the cream is heating up. This will prevent them from browning.
Do not place the sliced potatoes in water as that will begin to remove the starch needed to thicken the cream.
Roasted Garlic: Cut a ¼ inch off the top of the garlic. Wrap in foil or parchment. Bake for 30 to 40 minutes at 400 F degrees. Remove the cloves and mash with a fork. Make Ahead: Allow the baked casserole to cool fully. Cover the dish with foil and refrigerate. Let the dish sit at room temperature for at least 30 minutes then bake at 350 F degrees until heated through.