Preheat your oven to 375°F (190°C). Lightly grease a 9x13-inch baking dish. Ensure all potatoes are sliced to the same thickness (approximately 1/8 inch) and patted dry to remove excess starch.

Prepare the Roux: In a large, heavy-bottomed saucepan or Dutch oven, melt the ½ cup of butter over medium heat. Once the butter is fully melted and foamy, whisk in the ½ cup of all-purpose flour until a smooth paste, or roux, forms. Cook the roux for 2 to 3 minutes, stirring constantly, to cook out the raw flour taste. Do not let the roux brown.

Create the Cream Sauce: Slowly and gradually pour the warm milk into the roux, whisking constantly until the mixture is smooth and lump-free. Continue to whisk until the sauce thickens enough to coat the back of a spoon (about 5 to 7 minutes). Reduce the heat to low.

Season and Finish the Sauce: Remove the pan from the heat. Stir in the heavy cream, the 1 ½ teaspoons of kosher salt, the 1 teaspoon of black pepper, the garlic powder, onion powder, and ground nutmeg. Stir until fully combined. Next, add 1 cup of the shredded cheese to the sauce and stir until the cheese is completely melted and the sauce is uniform and creamy. Taste the sauce and adjust seasonings if necessary.

Layer the Potatoes: Arrange the sliced potatoes in the prepared 9x13-inch baking dish. Create the first layer by overlapping the slices slightly, covering the bottom of the dish entirely. Lightly sprinkle the layer with a pinch of the remaining salt and pepper. Pour approximately one-third of the cream sauce evenly over this first layer of potatoes, making sure the sauce flows between the gaps.

Complete the Layers: Repeat the layering process two more times: place the second layer of sliced potatoes, sprinkle with seasoning, and pour over another one-third of the cream sauce. Finally, place the last layer of potatoes and pour the remaining cream sauce over the top. Gently press down on the potatoes with the back of a spoon to ensure all slices are submerged in the sauce.

Initial Baking (Covered): Cover the baking dish tightly with aluminum foil. Place the dish into the preheated 375°F (190°C) oven and bake for 60 minutes. Baking covered ensures the potatoes cook fully and tenderize without the top crust drying out or burning.

Final Baking (Uncovered): After 60 minutes, carefully remove the foil. Sprinkle the remaining ½ cup of shredded cheese evenly over the top of the potatoes. Return the dish to the oven, uncovered, and continue baking for another 20 to 30 minutes, or until the potatoes are fork-tender, the sauce is bubbly, and the top crust is golden brown and caramelized.

Rest and Serve: Remove the scalloped potatoes from the oven and let them rest on a wire rack for at least 15 to 20 minutes before serving. Resting allows the sauce to fully set and absorb back into the layers, which prevents the dish from running apart when sliced.