Preheat the oven to 180°C.
Mash the bananas, then mix with the oats, vanilla and cinnamon and rest.
Make the filling. Peel then thinly slice the apple, and put them in a small saucepan. Add the sugar, vanilla, lemon juice and sugar, then simmer until the sugar has dissolved and the apple has softened. Set aside.
Add the flour, ground walnut to the banana & oat mixture, then combine until you get a dough.
Grease and line a baking dish, then press firmly half of the mixture into the base.
Spread the apple mixture on top, then crumble the remaining oat mixture over the apple, pressing down gently with your fingers.
Bake for about 25 minutes or until the top is golden.
Take the tin out of the oven and let it cool a bit, then transfer it to a wire rack.
When completely cool, slice it into squares or bars.