Preheat oven to 475. Peel, core and very thinly slice the apples: you should have approximately 1- half cups. Melt 3 tablespoons of the butter over medium low heat in a small fry pan, and saute the apples until they are just tender. Keep apples warm while preparing the batter.
Place a 9 or 25 cm cast-iron skillet in the oven to heat for at least 5 minutes the pan has to be very hot for this to work. When it is well heated, add the remaining 2 tablespoons of butter to melt and put the skillet back in the oven. The butter should be very hot buy not brown when you add the apples and the batter.
Place the flour, milk, vanilla, salt and nutmeg in a blender and whirl until smooth. Remove the skillet from the oven, quickly arrange the warm apple slices over the melted butter, and pour the batter evenly over all. Bake for 15 minutes. Reduce heat to 375 and bake 10 minutes longer. The pancake will puff and climb up the sides of the pan. Sprinkle with confectioner's sugar, then cut in wedges and serve with maple syrup.
Note: If you do not use apples, add quarter cup ( half stick) butter to the hot skillet.