Preheat the oven to 180°C and line a baking tin with parchment paper.
In a bowl, mash the bananas and blackberries with a fork until mostly smooth.
Add the grated apple, chia seeds and vanilla extract, and mix well.
Stir in the oats until fully combined. The mixture should be soft but not runny - add a little more oats if needed.
Fold in the chocolate chips.
Let the mixture rest for a few minutes so the oats can absorb some moisture.
Transfer to the prepared tin and spread evenly.
Bake for about 15-20 minutes (in the oven or air fryer), until lightly golden and set.
Allow to cool on a wire rack before cutting into bars or squares.