Preheat your oven to 375°F (190°C). Lightly grease a 9x13 inch baking dish with butter or non-stick spray.

Peel the potatoes and slice them very thinly, about 1/8-inch thick. A mandoline slicer is highly recommended for uniform slices, ensuring even cooking. If using a knife, take care to make them as consistent as possible. Set aside.

In a large saucepan or Dutch oven, melt the butter over medium heat. Add the minced garlic and sauté for 1 minute until fragrant, being careful not to brown it.

Whisk in the all-purpose flour and cook for 1-2 minutes, stirring constantly, to create a roux. It should be a pale, golden paste.

Gradually whisk in the warmed whole milk and heavy cream, a little at a time, until the sauce is smooth and no lumps remain. Continue to cook, stirring frequently, until the sauce thickens to a consistency similar to gravy, about 5-7 minutes.

Remove the saucepan from the heat. Stir in the salt, black pepper, onion powder, and nutmeg. Add the shredded Gruyère and sharp cheddar cheeses (reserving about 1/2 cup of cheddar for the topping). Stir until the cheeses are fully melted and the sauce is smooth and creamy.

Arrange one-third of the sliced potatoes evenly in the prepared baking dish, slightly overlapping them. Pour one-third of the cheese sauce over the potatoes, spreading it to cover.

Repeat with two more layers of potatoes and cheese sauce, ensuring the top layer of potatoes is fully covered with sauce.

Cover the baking dish tightly with aluminum foil.

Bake for 35 minutes. Then, remove the foil, sprinkle the reserved 1/2 cup of sharp cheddar cheese over the top, and continue baking uncovered for another 20-25 minutes, or until the potatoes are tender when pierced with a fork, and the top is golden brown and bubbly.

Once baked, remove the dish from the oven and let it rest for 10-15 minutes before serving. This allows the sauce to set and makes for easier serving.