Preheat the oven to 375 F (190 C). Grease a casserole dish with butter.
Place the apple slices, brown sugar, cinnamon and butter into a pan over low heat. Stir, cover with a lid and let the apples simmer for 10-15 minutes, stirring occasionally. The apples should soften and release their juices.
In the meantime, make the crumble topping. Place the flour, almond flour, oats, sugar and a pinch of salt in a large mixing bowl. Mix to combine.
Add the cubed cold butter and, using your fingertips, rub the butter into the dry ingredients to form a mixture that looks like coarse wet sand. Set the crumble aside.
Place the egg whites in a large mixing bowl, and, using an electric mixer, whisk them to stiff peaks. Reserve the whisk for the next step (no need to wash it).
Place the farmer's cheese, sour cream, sugar, melted butter, vanilla bean paste and egg yolks into a separate large mixing bowl. Using the electric hand mixer, beat the mixture until smooth and cohesive.
Carefully fold the egg whites into the farmer's cheese mixture, trying to retain as much air as possible.
Tip the stewed apples into a casserole dish. Then add the farmer;s cheese layer on top. And top everything wth the crumble mixture.
Bake it in the oven for 30 minutes. Enjoy it warm or at room temperature!