Preheat oven to 375°. Butter a baking dish well and set aside. Slice potatoes ¼" thick and place in water to keep from browning.
Heat butter over medium heat and add garlic. Sauce until fragrant about 1 minute. Add cream, milk, thyme, nutmeg, salt, and red pepper flakes. Bring to a boil and reduce heat to low. Let simmer 10 minutes. Remove from heat.
Layer ⅓rd of potatoes, slightly overlapping, in prepared dish then pour ⅓ of cream mixture over potatoes. Repeat with remaining potatoes and cream to make two more layers. Sprinkle gruyere over top.
Cover with foil and bake for 45 minutes. Uncover, sprinkle with parmesan and continue baking until potatoes are tender and top is golden, 15 to 20 minutes.
Recipe by Cooking TV Recipes at https://cookingtv-channel.com/potatoes-au-gratin/