Preheat your oven to 375°F (190°C) and grease a 9x9-inch baking dish with butter or cooking spray.

In a large bowl, combine the rolled oats, baking powder, cinnamon, and salt. Mix well.

In a separate bowl, whisk together the milk, mashed bananas, egg, maple syrup, and vanilla extract until smooth.

Pour the wet ingredients into the dry ingredients and stir until fully combined.

Gently fold in the blueberries and chopped nuts (if using).

Transfer the mixture into the prepared baking dish, spreading it evenly.

Bake in the preheated oven for 35-40 minutes, or until the top is golden brown and a toothpick inserted into the center comes out clean.

Allow the baked oatmeal to cool for a few minutes before serving. Serve warm or at room temperature.