Preheat your oven to 190°C/170°C(fan)/375°F/Gas 5.
Put all of the ingredients into a blender.
Blend until smooth.
Pour the batter into the greased baking dish.
Bake for 20 minutes.
Notes
Use a high powered blender to get the batter as smooth as possible.
If you don't have a blender, then you could use oat flour (or even just whole oats) instead, and mash the bananas and mix everything together. It would change the texture, and it wouldn't be cake like. But it would still be delicious.
To make this vegan, you can use flax egg and a plant based milk.
To ensure this recipe is gluten free, make sure to use certified gluten-free oats.