Heat the oven to 180°C. Line a small (22x22cm) baking tin with parchment paper.
In a mixing bowl, mash the bananas with a fork. Add the cinnamon, grated apple, oats, nuts and the oil. Mix until you get a thick mixture but not a dough. Add the chopped blackberries.
Transfer the mixture to the tin, spreading it out evenly, then bake for 25 minutes.
Leave it to cool in the tin for 5 minutes, then transfer it to a wire rack and let it cool completely.
Cut it into bars or squares.
Store the slices in a cake tin.