Preheat the oven to 160ºC . Press the cottage cheese through a sieve and drain. In a large mixing bowl, beat the egg yolks until light an foamy, then add the sugar slowly, continuing to beat until very light and smooth. Add the cottage cheese to the egg mixture, blending well, then add the sour cream, cornstarch, lemon rind, and walnuts (if desired). Stir until all ingredients are well blended and the mixture is smooth. In another large mixing bowl, beat the egg whites until they form soft peaks, then gently fold them into the batter. Pour the mixture into the prepared crust and bake for about 1 hour. Cool to room temperature before serving.