Whisk milk, eggs, sugar, vanilla, salt, cinnamon, and nutmeg in large bowl until well blended. Add flour and whisk until batter is smooth.
Place butter in a glass pie dish. Place dish in oven until butter melts, about 5 minutes. Remove dish from oven.
Place apple slices in overlapping rows atop melted butter in baking dish (they don’t need to be orderly, particularly if you’d like a rustic look).
Return to oven and bake until apples begin to soften slightly and butter is bubbling and beginning to brown around edges of dish, about 10 minutes.
Pour batter over apples in dish and sprinkle with brown sugar. Bake pancake until puffed and brown, about 20 minutes. Sprinkle with powdered sugar, if desired. Serve warm.