Preheat your oven to 375°F (190°C). Generously grease your baking dish with butter to prevent sticking. This ensures the edges get nice and crispy.

Peel the potatoes and slice them into 1/8-inch thick rounds. Use a mandoline if you have one for even pieces. Uniform slices help the potatoes cook at the same rate.

Melt butter in a saucepan over medium heat. Add the minced onion and garlic. Cook for about 3 minutes until they are soft, fragrant, and translucent.

Whisk the flour into the butter and onion mixture. Cook it for 1 minute to remove the raw flour taste. Keep stirring so it does not burn or brown.

Slowly pour in the milk and heavy cream while whisking constantly. Bring the mixture to a gentle simmer. It should thicken slightly into a smooth, creamy sauce.

Stir in the salt, black pepper, and nutmeg. The nutmeg adds a subtle warmth that is classic in French-style cream sauces. Remove the pan from the heat.

Arrange half of the potato slices in the prepared baking dish. Pour half of the cream sauce over them. Repeat with the remaining potatoes and the rest of the sauce.

Cover the dish tightly with aluminum foil. Bake for 40 minutes. Covering the dish traps steam, which makes the potatoes tender and prevents the top from burning.

Remove the foil and sprinkle cheese over the top if desired. Bake for another 20 minutes. The top should be golden brown and the sauce should be bubbly.

Take the dish out of the oven. Let it sit for 10 minutes before serving. This allows the sauce to set so it isn't too runny when scooped.