Preheat the oven to 190 C / 170 C fan / Gas 5. Line a baking tray with baking parchment.
Put oatbran in small food processor to make it finer. It doesn't have to be like powder but if not blended the cookies can taste bitty.
Crack the egg into a bowl. Mix with the oatbran.
Finely slice the ripe banana and then cut each slice in half. Or alternatively cut the banana down the middle lengthways and then slice finely as this may be easier.
Add the banana and raisins to the oatbran mixture. Using a soft spatula mix together until each slice of banana is coated. If the consistency is quite dry this could be down to the banana being big or the egg being small - but this doesn't matter!
Using two spoons place 6 to 8 spoonfuls at ample distance from each other on the baking tray. Flatten the cookies by gently squishing them with the spatula or the back of the spoon.
Bake in the preheated oven until the edges and bottoms are slightly golden brown, about 18 to 20 minutes.
Place the golden brown cookies on a wire rack and cool completely before eating.