Vegetable oil, for the baking dish 1 cup low-fat milk 1/2 cup half-and-half 1 tablespoon all-purpose flour 1 tablespoon unsalted butter 1 tablespoon fresh thyme leaves, or 2 teaspoons dried 1/2 teaspoon ground nutmeg Salt and freshly ground black pepper 3/4 cup grated Gruyère cheese 1/2 cup freshly grated Parmesan cheese 4 medium sweet potatoes, peeled and sliced very thinly (about 1/8 inch thick), like a sweet potato chip (a mandoline helps) 1/2 cup Italian bread crumbs
- Preheat the oven to 400°F. Lightly grease a 4-quart glass lasagna-style baking dish with vegetable oil.
- In a saucepan, whisk together the milk, half-and-half, flour, butter, thyme, and nutmeg and season with salt and pepper. Heat over medium heat, allowing the milk mixture to thicken slightly, like a looser roux.
- In a medium bowl, mix the Gruyère and Parmesan cheeses.
- Arrange the potatoes in tight, thin layers in the prepared baking dish, pouring a bit of the cream mixture over each layer and sprinkling a couple of tablespoons of cheeses on top. Aim for five or six layers and reserve some of the cheese to top the last layer.
- Top with the reserved cheese, sprinkle with the bread crumbs, and season with salt and pepper. Cover and bake for 30 minutes, then remove the cover and bake until the top browns and a knife inserted into the potatoes comes out easily, 10 to 15 minutes more. Serve and enjoy!