Preheat your oven to 350°F (175°C).
In a large mixing bowl, combine the rolled oats, baking powder, ground cinnamon, and half of the chopped walnuts (1/4 cup). Stir until well mixed.
In another bowl, whisk together the milk, maple syrup, beaten egg, melted coconut oil (or butter), and vanilla extract until fully combined.
Pour the wet ingredients into the dry oat mixture. Stir until everything is well incorporated.
Gently fold in the sliced bananas and mixed berries, making sure they are evenly distributed throughout the oatmeal mixture.
Grease your baking dish with a little coconut oil or butter. Pour the oatmeal mixture into the prepared dish, spreading it out evenly.
Sprinkle the remaining 1/4 cup of chopped walnuts on top for an added crunch.
Place the baking dish in the preheated oven and bake for 30-35 minutes, or until the top is golden brown and the oatmeal is set.
Once done, remove from the oven and allow it to cool for a few minutes before serving.
