First, preheat a 10-inch cast-iron skillet in an oven set to 425ºF. Meanwhile, in a blender, combine the eggs, milk, flour, salt and nutmeg. Cover the blender and process the ingredients until the mixture is smooth.
Add the butter to the hot skillet and return the skillet to the oven until the butter bubbles. Then, pour the batter into the skillet. Bake the pancake, uncovered, until it puffs up and the edges are browned and crisp, which will take about 20 minutes.
As the pancake bakes, prepare the topping. In a skillet, combine the apples, butter and sugar and cook, stirring often, over medium heat. Cook this mixture until the apples are tender, then spoon the topping onto the baked pancake. Sprinkle the pancake with confectioners' sugar, then cut and serve immediately.
