Heat oven to 425 degrees. Lightly oil a 10-inch cast-iron skillet. Toss apples with both sugars and cinnamon in a large bowl.
Mix milk and eggs in a blender or food processor; add flour and salt and mix to combine.
Melt butter in prepared skillet over medium heat. Add apple mixture; cook, stirring often, until sugar melts, about 5 minutes. Remove from heat and pour batter over.
Bake until pancake is puffed and golden, 20 to 30 minutes. Serve at once.