Nutrition per serving may change if servings are adjusted.
2 tablespoons unsalted butter
6 cups sliced leeks
1 tablespoon water
¾ teaspoon salt, divided
1½ cups whole milk
½ cup grated Parmesan cheese
½ cup chopped fresh Italian parsley, divided
2 tablespoons all-purpose flour
1 teaspoon ground pepper
½ teaspoon ground nutmeg
2½-3 pounds Yukon gold potatoes, peeled and thinly sliced
8 ounces red-skin sweet potatoes, peeled and thinly sliced
⅓ cup grated reduced-fat sharp Cheddar cheese
Preheat oven to 350°F. Lightly coat a 3-quart rectangular baking dish with cooking spray; set aside.
Melt butter in a large saucepan over medium heat. Add leeks, the water, and ¼ teaspoon of salt. Cook for 10 to 15 minutes or until tender and most of the liquid has evaporated, stirring frequently. Remove from the heat.
Whisk together milk, Parmesan, ¼ cup of the parsley, the flour, pepper, nutmeg, and the remaining ½ teaspoon salt in a very large bowl. Add all potato slices; toss to coat.
Layer half of the coated potatoes in the prepared baking dish. Top with half of the leeks. Repeat layers. Pour any remaining milk mixture over the potatoes.
Coat a large piece of foil with cooking spray; cover the baking dish with the foil, placing the cooking spray-coated side down. Bake for 45 minutes. Increase the oven temperature to 425°F. Sprinkle with Cheddar. Bake, uncovered, in the 425°F oven for 20 to 30 minutes more or until the potatoes are tender and the top is golden brown. Let stand for 15 minutes before serving. Sprinkle with the remaining ¼ cup parsley.