Preheat your oven to 350°F (175°C).
Grease a 9x13 inch baking dish with butter.
In a large mixing bowl, combine the heavy cream, whole milk, minced garlic, salt, black pepper, nutmeg (if using), and thyme. Whisk until well combined.
Layer half of the sliced potatoes evenly in the bottom of the greased baking dish. Sprinkle with half of the cheddar cheese and half of the Parmesan cheese.
Pour half of the cream mixture over the first layer of potatoes and cheese.
Repeat the layering process with the remaining potatoes, cheeses, and cream mixture.
Cover the baking dish with aluminum foil and bake in the preheated oven for 40 minutes.
Remove the foil and bake for an additional 20 minutes, or until the potatoes are tender and the top is golden brown and bubbly.
Let the scalloped potatoes rest for 10 minutes before serving.