Peel and quarter the apples, remove the core and cut into leaves. Drizzle with lemon juice and mix in the sour cream.
Beat the butter with the mixer until foamy, gradually stir in half of the sugar and the egg yolks. Beat the egg whites until stiff, fold in the remaining sugar and hazelnuts and mix into the yolk mixture.
Fill a greased casserole dish alternately with the mixture and with apples in layers and bake in the oven at 200 ° for approx. 40 minutes until light brown (with circulating air approx. 175 °). Serve hot.