Peel the potatoes and slice them thinly. A mandolin can help make uniform slices.

In a skillet, melt the butter over medium heat. Add the sliced onions and cook until soft and translucent, about 5 minutes. Stir in the minced garlic for just a minute.

In a greased 9x13-inch baking dish, layer sliced potatoes, sautéed onions, and sprinkle cheese over the top. Repeat until the dish is full, finishing with a layer of cheese.

In a mixing bowl, combine the heavy cream, salt, pepper, and nutmeg if using. Pour this mixture over the layered potatoes and cheese.

Cover with aluminum foil and bake in a preheated oven at 375°F (190°C) for about 45 minutes. Remove the foil and bake for an additional 15 to 20 minutes until potatoes are tender and cheese is bubbly.

Allow the scalloped potatoes to sit for about 10 minutes before serving.