Preheat oven to 190°C and line a square baking dish.
Add 80 g oats to a blender or food processor and blend into a fine flour. Leave the remaining 120 g oats whole for texture.
In a large bowl, mash the bananas until smooth. Add the whole oats, oat flour, ground almonds, baking soda, baking powder, cinnamon, salt, Greek yogurt, eggs, maple syrup, milk, and vanilla extract. Add walnuts if using. Mix until fully combined.
Pour the mixture into the baking dish and smooth the top. Sprinkle over the chopped walnuts.
Bake for 25–30 mins until golden and set in the middle.
Leave to cool slightly before slicing. Serve topped with fresh banana and Greek yogurt.
