In a bowl, mix oat flakes, baking powder, and cinnamon.
In a separate bowl, whisk egg yolks until smooth.
In another bowl, beat egg whites with a pinch of salt until stiff peaks form.
Gently fold half of the egg whites into the oat mixture, then fold in the remaining half.
Peel and chop the apples, then fold them into the oat mixture.
Grease a baking dish with butter or oil.
Pour the mixture into the baking dish, spreading it evenly.
Top with chopped walnuts for decoration.
Bake for 35-40 minutes until golden brown and set.
Allow the bake to cool before serving.
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