1. In a 10 inch oven–proof skillet ( I prefer my cast iron skillet) melt butter and add sugar, sliced apples and lemon juice. Cook uncovered on medium-high heat about 10 minutes until apples have absorbed liquid and liquids are reduced almost in half. Remove 1/2 of apples and set aside.
2. In a bowl beat egg yolks until thick; beat in sour cream and vanilla. In a separate bowl, with clean beaters, beat whites until almost stiff and then beat in sugar until whites are stiff. Fold whites into yolk mixture.
3. Pour batter over the apples in the skillet and arrange remaining apple slices over the top of the batter. Bake in a preheated 350 degree F. oven until puffed and golden, about 25 minutes. Carefully remove pan to cooling rack. Dust puff with powdered sugar and a grating of nutmeg.
4. Serve immediately. This has the texture of a soufflé and will deflate. Cut into 6 to 8 pie-like sections and remove from pan with a spatula. The pieces will hold their shape.