Preheat Oven : Preheat your oven to 375°F (190°C). Butter a 9x13-inch baking dish.
Prepare Potatoes : Peel and thinly slice potatoes (about 1/8-inch thick) for even cooking.
Make Cream Sauce : In a medium saucepan, melt butter over medium heat. Add minced garlic and sauté 1 minute until fragrant. Add heavy cream, milk, salt, black pepper, and nutmeg. Stir and heat until warm, but do not boil.
Layer Potatoes : Arrange a layer of sliced potatoes evenly in the prepared baking dish. Repeat until all potatoes are layered.
Pour Sauce : Pour the warm cream sauce over the layered potatoes, ensuring it seeps through the layers.
Add Cheese : Sprinkle shredded cheddar and Parmesan evenly over the top.
Bake Covered : Cover the dish with foil and bake for 45 minutes.
Bake Uncovered : Remove foil and bake an additional 15 minutes, or until the top is golden brown and bubbly.