Preheat your oven to 350°F (175°C).
In a large mixing bowl, combine the rolled oats, baking powder, cinnamon, and salt.
In another bowl, whisk together the milk, maple syrup, mashed banana, and vanilla extract.
Pour the wet ingredients into the dry ingredients and mix until well combined.
Fold in the mixed berries and nuts if using.
Grease a baking dish (about 8x8 inches) with cooking spray or butter.
Pour the oatmeal mixture into the prepared dish, spreading it evenly.
Bake for 30 minutes, or until set and lightly golden on top.
Allow to cool for a few minutes before serving.
Serve warm, topped with additional fruit or a drizzle of honey if desired.