1 large tart-sweet apple, such as Cortland, peeled, cored, and thinly sliced
Juice and grated zest of 1 large lemon
4 large eggs, separated
½ cup whole milk
2 tablespoons sugar
2 tablespoons cornstarch
½ teaspoon salt
2 tablespoons unsalted butter
Confectioners’ sugar, to garnish
Preheat your oven to 400°F. Position the rack in the middle of the oven. Heat a 10-inch cast iron skillet over medium heat until hot, about 3 minutes.
In a bowl, combine the apple slices with 1 tablespoon of the lemon juice and set aside. In a large bowl, beat together the egg yolks, milk, sugar, cornstarch, salt, and lemon zest. In a separate large bowl, whisk the egg whites into soft peaks. Using a flexible spatula, gently fold the yolks into the whites until just mixed.
Add the butter to the heated skillet. When the butter foams, add the apples, turning to coat them with butter, and then arrange them in a single layer. Pour the egg mixture evenly over the apples. Partially cover, and cook for 10 minutes. Uncover the skillet, transfer it to the oven, and bake for about 15 minutes, or until the bottom of the pfannkuchen is nicely browned and it is set in the middle.
Remove, sprinkle with the confectioners’ sugar and remaining lemon juice, and serve immediately.