Cheesy Baked Potato Casserole with Creamy Onion Sauce

Method

1. Preheat the oven to 375°F with a rack in the middle. Lightly grease a 9x13-inch baking dish with nonstick spray and set aside.
2. In a medium pot or a wide skillet with tall sides, melt the butter over medium-low heat. Add the onion and cook, stirring often, for 8 to 10 minutes until soft and translucent. Add the garlic and cook for another minute until fragrant. Turn off the heat.
3. Stir in the milk, 1 teaspoon of salt, and the pepper. Set the mixture aside to cool slightly.
4. Peel the potatoes and slice them very thinly, about ⅛ to ¼-inch thick, using a mandoline or a sharp knife. Place the slices in a large bowl, sprinkle with the flour and the remaining 1 teaspoon of salt, and toss with your hands to coat evenly.
5. Spread one-third of the potato slices in the prepared dish. Spoon 1 cup of the milk mixture over the top and sprinkle with ½ cup of the shredded cheddar. Repeat twice more, ending with a final layer of cheese on top.
6. Cover the dish with a sheet of aluminum foil that has been lightly misted with nonstick spray (spray side down). Bake for 35 minutes, then remove the foil and bake for another 25 minutes, until the potatoes are tender when pierced with a fork.
7. Take the dish out of the oven and sprinkle with the Parmesan. Turn the oven to broil and return the dish for 1 to 3 minutes, until the cheese is golden and bubbly. Watch carefully to avoid burning.
8. Let the potatoes stand at room temperature for 20 to 25 minutes before serving, so the sauce thickens and sets.